diff --git a/.flowconfig b/.flowconfig
new file mode 100644
index 00000000..69ec8356
--- /dev/null
+++ b/.flowconfig
@@ -0,0 +1,23 @@
+[ignore]
+
+[include]
+
+[libs]
+
+[lints]
+all=warn
+
+[options]
+emoji=true
+include_warnings=true
+
+[strict]
+nonstrict-import
+sketchy-null
+unclear-type
+unsafe-getters-setters
+untyped-import
+untyped-type-import
+
+[version]
+^0.82.0
diff --git a/.gitignore b/.gitignore
index 11894ca3..7d7e2bc1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,13 +1,7 @@
-# Coverage directory used by tools like istanbul
-coverage
+# Coverage directory
 .nyc_output
 
 # Dependency directory
-# Commenting this out is preferred by some people, see
-# https://www.npmjs.org/doc/misc/npm-faq.html#should-i-check-my-node_modules-folder-into-git-
 node_modules
 
-# Users Environment Variables
-.lock-wscript
-
 .DS_Store
diff --git a/README.md b/README.md
index 887bf9f2..4bb6add4 100644
--- a/README.md
+++ b/README.md
@@ -91,7 +91,7 @@ You can additionally, pass options to `config`.
 
 Default: `path.resolve(process.cwd(), '.env')`
 
-You can specify a custom path if your file containing environment variables is
+You may specify a custom path if your file containing environment variables is
 named or located differently.
 
 ```js
@@ -109,6 +109,16 @@ using this option.
 require('dotenv').config({ encoding: 'base64' })
 ```
 
+#### Debug
+
+Default: `false`
+
+You may turn on logging to help debug why certain keys or values are not being set as you expect.
+
+```js
+require('dotenv').config({ debug: process.env.DEBUG })
+```
+
 ## Parse
 
 The engine which parses the contents of your file containing environment
@@ -122,6 +132,22 @@ const config = dotenv.parse(buf) // will return an object
 console.log(typeof config, config) // object { BASIC : 'basic' }
 ```
 
+### Options
+
+#### Debug
+
+Default: `false`
+
+You may turn on logging to help debug why certain keys or values are not being set as you expect.
+
+```js
+const dotenv = require('dotenv')
+const buf = Buffer.from('hello world')
+const opt = { debug: true }
+const config = dotenv.parse(buf, opt)
+// expect a debug message because the buffer is not in KEY=VAL form
+```
+
 ### Rules
 
 The parsing engine currently supports the following rules:
diff --git a/config.js b/config.js
index 8ac2a360..77641220 100644
--- a/config.js
+++ b/config.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 (function () {
   require('./lib/main').config(
     require('./lib/cli-options')(process.argv)
diff --git a/flow-typed/npm/sinon_vx.x.x.js b/flow-typed/npm/sinon_vx.x.x.js
new file mode 100644
index 00000000..183358be
--- /dev/null
+++ b/flow-typed/npm/sinon_vx.x.x.js
@@ -0,0 +1,361 @@
+// flow-typed signature: fd98cafe7bb28287e51ca85d49a17379
+// flow-typed version: <<STUB>>/sinon_v6.3.5/flow_v0.82.0
+
+/**
+ * This is an autogenerated libdef stub for:
+ *
+ *   'sinon'
+ *
+ * Fill this stub out by replacing all the `any` types.
+ *
+ * Once filled out, we encourage you to share your work with the
+ * community by sending a pull request to:
+ * https://github.com/flowtype/flow-typed
+ */
+
+declare module 'sinon' {
+  declare module.exports: any;
+}
+
+/**
+ * We include stubs for each file inside this npm package in case you need to
+ * require those files directly. Feel free to delete any files that aren't
+ * needed.
+ */
+declare module 'sinon/lib/sinon' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/assert' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/behavior' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/blob' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/call' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/collect-own-methods' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/color' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/create-sandbox' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/default-behaviors' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/fake' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/match' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/mock-expectation' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/mock' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/sandbox' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/spy-formatters' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/spy' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/stub-entire-object' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/stub-non-function-property' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/stub' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/throw-on-falsy-object' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/called-in-order' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/deep-equal' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/default-config' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/deprecated' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/every' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/extend' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/format' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/function-to-string' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/get-config' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/get-next-tick' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/get-property-descriptor' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/is-es-module' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/is-non-existent-own-property' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/is-property-configurable' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/is-restorable' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/iterable-to-string' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/next-tick' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/order-by-first-call' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/restore' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/times-in-words' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/typeOf' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/use-promise-library' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/walk' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/core/wrap-method' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/lib/sinon/util/fake_timers' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/pkg/sinon-esm' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/pkg/sinon-no-sourcemaps' {
+  declare module.exports: any;
+}
+
+declare module 'sinon/pkg/sinon' {
+  declare module.exports: any;
+}
+
+// Filename aliases
+declare module 'sinon/lib/sinon.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon'>;
+}
+declare module 'sinon/lib/sinon/assert.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/assert'>;
+}
+declare module 'sinon/lib/sinon/behavior.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/behavior'>;
+}
+declare module 'sinon/lib/sinon/blob.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/blob'>;
+}
+declare module 'sinon/lib/sinon/call.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/call'>;
+}
+declare module 'sinon/lib/sinon/collect-own-methods.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/collect-own-methods'>;
+}
+declare module 'sinon/lib/sinon/color.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/color'>;
+}
+declare module 'sinon/lib/sinon/create-sandbox.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/create-sandbox'>;
+}
+declare module 'sinon/lib/sinon/default-behaviors.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/default-behaviors'>;
+}
+declare module 'sinon/lib/sinon/fake.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/fake'>;
+}
+declare module 'sinon/lib/sinon/match.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/match'>;
+}
+declare module 'sinon/lib/sinon/mock-expectation.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/mock-expectation'>;
+}
+declare module 'sinon/lib/sinon/mock.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/mock'>;
+}
+declare module 'sinon/lib/sinon/sandbox.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/sandbox'>;
+}
+declare module 'sinon/lib/sinon/spy-formatters.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/spy-formatters'>;
+}
+declare module 'sinon/lib/sinon/spy.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/spy'>;
+}
+declare module 'sinon/lib/sinon/stub-entire-object.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/stub-entire-object'>;
+}
+declare module 'sinon/lib/sinon/stub-non-function-property.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/stub-non-function-property'>;
+}
+declare module 'sinon/lib/sinon/stub.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/stub'>;
+}
+declare module 'sinon/lib/sinon/throw-on-falsy-object.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/throw-on-falsy-object'>;
+}
+declare module 'sinon/lib/sinon/util/core/called-in-order.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/called-in-order'>;
+}
+declare module 'sinon/lib/sinon/util/core/deep-equal.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/deep-equal'>;
+}
+declare module 'sinon/lib/sinon/util/core/default-config.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/default-config'>;
+}
+declare module 'sinon/lib/sinon/util/core/deprecated.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/deprecated'>;
+}
+declare module 'sinon/lib/sinon/util/core/every.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/every'>;
+}
+declare module 'sinon/lib/sinon/util/core/extend.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/extend'>;
+}
+declare module 'sinon/lib/sinon/util/core/format.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/format'>;
+}
+declare module 'sinon/lib/sinon/util/core/function-to-string.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/function-to-string'>;
+}
+declare module 'sinon/lib/sinon/util/core/get-config.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/get-config'>;
+}
+declare module 'sinon/lib/sinon/util/core/get-next-tick.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/get-next-tick'>;
+}
+declare module 'sinon/lib/sinon/util/core/get-property-descriptor.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/get-property-descriptor'>;
+}
+declare module 'sinon/lib/sinon/util/core/is-es-module.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/is-es-module'>;
+}
+declare module 'sinon/lib/sinon/util/core/is-non-existent-own-property.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/is-non-existent-own-property'>;
+}
+declare module 'sinon/lib/sinon/util/core/is-property-configurable.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/is-property-configurable'>;
+}
+declare module 'sinon/lib/sinon/util/core/is-restorable.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/is-restorable'>;
+}
+declare module 'sinon/lib/sinon/util/core/iterable-to-string.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/iterable-to-string'>;
+}
+declare module 'sinon/lib/sinon/util/core/next-tick.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/next-tick'>;
+}
+declare module 'sinon/lib/sinon/util/core/order-by-first-call.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/order-by-first-call'>;
+}
+declare module 'sinon/lib/sinon/util/core/restore.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/restore'>;
+}
+declare module 'sinon/lib/sinon/util/core/times-in-words.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/times-in-words'>;
+}
+declare module 'sinon/lib/sinon/util/core/typeOf.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/typeOf'>;
+}
+declare module 'sinon/lib/sinon/util/core/use-promise-library.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/use-promise-library'>;
+}
+declare module 'sinon/lib/sinon/util/core/walk.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/walk'>;
+}
+declare module 'sinon/lib/sinon/util/core/wrap-method.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/core/wrap-method'>;
+}
+declare module 'sinon/lib/sinon/util/fake_timers.js' {
+  declare module.exports: $Exports<'sinon/lib/sinon/util/fake_timers'>;
+}
+declare module 'sinon/pkg/sinon-esm.js' {
+  declare module.exports: $Exports<'sinon/pkg/sinon-esm'>;
+}
+declare module 'sinon/pkg/sinon-no-sourcemaps.js' {
+  declare module.exports: $Exports<'sinon/pkg/sinon-no-sourcemaps'>;
+}
+declare module 'sinon/pkg/sinon.js' {
+  declare module.exports: $Exports<'sinon/pkg/sinon'>;
+}
diff --git a/flow-typed/npm/tap_vx.x.x.js b/flow-typed/npm/tap_vx.x.x.js
new file mode 100644
index 00000000..069b481a
--- /dev/null
+++ b/flow-typed/npm/tap_vx.x.x.js
@@ -0,0 +1,144 @@
+// flow-typed signature: babbe08a031ef4f7eb2f8d9812098a53
+// flow-typed version: <<STUB>>/tap_v12.0.1/flow_v0.82.0
+
+/**
+ * This is an autogenerated libdef stub for:
+ *
+ *   'tap'
+ *
+ * Fill this stub out by replacing all the `any` types.
+ *
+ * Once filled out, we encourage you to share your work with the
+ * community by sending a pull request to:
+ * https://github.com/flowtype/flow-typed
+ */
+
+declare module 'tap' {
+  declare module.exports: any;
+}
+
+/**
+ * We include stubs for each file inside this npm package in case you need to
+ * require those files directly. Feel free to delete any files that aren't
+ * needed.
+ */
+declare module 'tap/bin/mochatap' {
+  declare module.exports: any;
+}
+
+declare module 'tap/bin/run' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/base' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/clean-yaml-object' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/diags' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/extra-from-error' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/mocha' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/obj-to-yaml' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/parse-test-args' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/point' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/snapshot' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/spawn' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/stack' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/stdin' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/synonyms' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/tap' {
+  declare module.exports: any;
+}
+
+declare module 'tap/lib/test' {
+  declare module.exports: any;
+}
+
+// Filename aliases
+declare module 'tap/bin/mochatap.js' {
+  declare module.exports: $Exports<'tap/bin/mochatap'>;
+}
+declare module 'tap/bin/run.js' {
+  declare module.exports: $Exports<'tap/bin/run'>;
+}
+declare module 'tap/lib/base.js' {
+  declare module.exports: $Exports<'tap/lib/base'>;
+}
+declare module 'tap/lib/clean-yaml-object.js' {
+  declare module.exports: $Exports<'tap/lib/clean-yaml-object'>;
+}
+declare module 'tap/lib/diags.js' {
+  declare module.exports: $Exports<'tap/lib/diags'>;
+}
+declare module 'tap/lib/extra-from-error.js' {
+  declare module.exports: $Exports<'tap/lib/extra-from-error'>;
+}
+declare module 'tap/lib/mocha.js' {
+  declare module.exports: $Exports<'tap/lib/mocha'>;
+}
+declare module 'tap/lib/obj-to-yaml.js' {
+  declare module.exports: $Exports<'tap/lib/obj-to-yaml'>;
+}
+declare module 'tap/lib/parse-test-args.js' {
+  declare module.exports: $Exports<'tap/lib/parse-test-args'>;
+}
+declare module 'tap/lib/point.js' {
+  declare module.exports: $Exports<'tap/lib/point'>;
+}
+declare module 'tap/lib/snapshot.js' {
+  declare module.exports: $Exports<'tap/lib/snapshot'>;
+}
+declare module 'tap/lib/spawn.js' {
+  declare module.exports: $Exports<'tap/lib/spawn'>;
+}
+declare module 'tap/lib/stack.js' {
+  declare module.exports: $Exports<'tap/lib/stack'>;
+}
+declare module 'tap/lib/stdin.js' {
+  declare module.exports: $Exports<'tap/lib/stdin'>;
+}
+declare module 'tap/lib/synonyms.js' {
+  declare module.exports: $Exports<'tap/lib/synonyms'>;
+}
+declare module 'tap/lib/tap.js' {
+  declare module.exports: $Exports<'tap/lib/tap'>;
+}
+declare module 'tap/lib/test.js' {
+  declare module.exports: $Exports<'tap/lib/test'>;
+}
diff --git a/lib/cli-options.js b/lib/cli-options.js
index 36ecdf61..ad69fabd 100644
--- a/lib/cli-options.js
+++ b/lib/cli-options.js
@@ -1,6 +1,8 @@
-const re = /^dotenv_config_(encoding|path)=(.+)$/
+/* @flow */
 
-module.exports = function optionMatcher (args) {
+const re = /^dotenv_config_(encoding|path|debug)=(.+)$/
+
+module.exports = function optionMatcher (args /*: Array<string> */) {
   return args.reduce(function (acc, cur) {
     const matches = cur.match(re)
     if (matches) {
diff --git a/lib/main.js b/lib/main.js
index ae61a3d1..4da2961b 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -1,16 +1,40 @@
+/* @flow */
+/*::
+
+type DotenvParseOptions = {
+  debug?: boolean
+}
+
+// keys and values from src
+type DotenvParseOutput = { [string]: string }
+
+type DotenvConfigOptions = {
+  path?: string, // path to .env file
+  encoding?: string, // encoding of .env file
+  debug?: string // turn on logging for debugging purposes
+}
+
+type DotenvConfigOutput = {
+  parsed?: DotenvParseOutput,
+  error?: Error
+}
+
+*/
+
 const fs = require('fs')
 const path = require('path')
 
-/*
- * Parses a string or buffer into an object
- * @param {(string|Buffer)} src - source to be parsed
- * @returns {Object} keys and values from src
-*/
-function parse (src) {
+function log (message /*: string */) {
+  console.log(`[dotenv][DEBUG] ${message}`)
+}
+
+// Parses src into an Object
+function parse (src /*: string | Buffer */, options /*: ?DotenvParseOptions */) /*: DotenvParseOutput */ {
+  const debug = Boolean(options && options.debug)
   const obj = {}
 
   // convert Buffers before splitting into lines and processing
-  src.toString().split('\n').forEach(function (line) {
+  src.toString().split('\n').forEach(function (line, idx) {
     // matching "KEY' and 'VAL' in 'KEY=VAL'
     const keyValueArr = line.match(/^\s*([\w.-]+)\s*=\s*(.*)?\s*$/)
     // matched?
@@ -30,39 +54,41 @@ function parse (src) {
       value = value.replace(/(^['"]|['"]$)/g, '').trim()
 
       obj[key] = value
+    } else if (debug) {
+      log(`did not match key and value when parsing line ${idx + 1}: ${line}`)
     }
   })
 
   return obj
 }
 
-/*
- * Main entry point into dotenv. Allows configuration before loading .env
- * @param {Object} options - options for parsing .env file
- * @param {string} [options.path=.env] - path to .env file
- * @param {string} [options.encoding=utf8] - encoding of .env file
- * @returns {Object} parsed object or error
-*/
-function config (options) {
+// Populates process.env from .env file
+function config (options /*: ?DotenvConfigOptions */) /*: DotenvConfigOutput */ {
   let dotenvPath = path.resolve(process.cwd(), '.env')
-  let encoding = 'utf8'
+  let encoding /*: string */ = 'utf8'
+  let debug = false
 
   if (options) {
-    if (options.path) {
+    if (options.path != null) {
       dotenvPath = options.path
     }
-    if (options.encoding) {
+    if (options.encoding != null) {
       encoding = options.encoding
     }
+    if (options.debug != null) {
+      debug = true
+    }
   }
 
   try {
     // specifying an encoding returns a string instead of a buffer
-    const parsed = parse(fs.readFileSync(dotenvPath, { encoding }))
+    const parsed = parse(fs.readFileSync(dotenvPath, { encoding }), { debug })
 
     Object.keys(parsed).forEach(function (key) {
       if (!process.env.hasOwnProperty(key)) {
         process.env[key] = parsed[key]
+      } else if (debug) {
+        log(`"${key}" is already defined in \`process.env\` and will not be overwritten`)
       }
     })
 
diff --git a/package-lock.json b/package-lock.json
index ee758862..9ae1c6e7 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -50,13 +50,10 @@
       }
     },
     "@sinonjs/samsam": {
-      "version": "2.1.1",
-      "resolved": "https://registry.npmjs.org/@sinonjs/samsam/-/samsam-2.1.1.tgz",
-      "integrity": "sha512-7oX6PXMulvdN37h88dvlvRyu61GYZau40fL4wEZvPEHvrjpJc3lDv6xDM5n4Z0StufUVB5nDvVZUM+jZHdMOOQ==",
-      "dev": true,
-      "requires": {
-        "array-from": "^2.1.1"
-      }
+      "version": "2.1.2",
+      "resolved": "https://registry.npmjs.org/@sinonjs/samsam/-/samsam-2.1.2.tgz",
+      "integrity": "sha512-ZwTHAlC9akprWDinwEPD4kOuwaYZlyMwVJIANsKNC3QVp0AHB04m7RnB4eqeWfgmxw8MGTzS9uMaw93Z3QcZbw==",
+      "dev": true
     },
     "acorn": {
       "version": "5.7.3",
@@ -237,7 +234,7 @@
       "dependencies": {
         "chalk": {
           "version": "1.1.3",
-          "resolved": "http://registry.npmjs.org/chalk/-/chalk-1.1.3.tgz",
+          "resolved": "https://registry.npmjs.org/chalk/-/chalk-1.1.3.tgz",
           "integrity": "sha1-qBFcVeSnAv5NFQq9OHKCKn4J/Jg=",
           "dev": true,
           "requires": {
@@ -250,7 +247,7 @@
         },
         "strip-ansi": {
           "version": "3.0.1",
-          "resolved": "http://registry.npmjs.org/strip-ansi/-/strip-ansi-3.0.1.tgz",
+          "resolved": "https://registry.npmjs.org/strip-ansi/-/strip-ansi-3.0.1.tgz",
           "integrity": "sha1-ajhfuIU9lS1f8F0Oiq+UJ43GPc8=",
           "dev": true,
           "requires": {
@@ -1247,7 +1244,7 @@
     },
     "external-editor": {
       "version": "2.2.0",
-      "resolved": "http://registry.npmjs.org/external-editor/-/external-editor-2.2.0.tgz",
+      "resolved": "https://registry.npmjs.org/external-editor/-/external-editor-2.2.0.tgz",
       "integrity": "sha512-bSn6gvGxKt+b7+6TKEv1ZycHleA7aHhRHyAqJyp5pbUFuYYNIzpZnQDk7AsYckyWdEnTeAnay0aCy2aV6iTk9A==",
       "dev": true,
       "requires": {
@@ -1429,6 +1426,12 @@
         "write": "^0.2.1"
       }
     },
+    "flow-bin": {
+      "version": "0.82.0",
+      "resolved": "https://registry.npmjs.org/flow-bin/-/flow-bin-0.82.0.tgz",
+      "integrity": "sha512-D7ViTCVJSVv19CB6dFWS9k2iKQlavtkRXn9el0ofVTTpGuybe+EPE8DZwdyohzEt6wRhHV8gwkteWvxdcVuOzg==",
+      "dev": true
+    },
     "for-in": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/for-in/-/for-in-1.0.2.tgz",
@@ -1805,7 +1808,7 @@
     },
     "is-builtin-module": {
       "version": "1.0.0",
-      "resolved": "http://registry.npmjs.org/is-builtin-module/-/is-builtin-module-1.0.0.tgz",
+      "resolved": "https://registry.npmjs.org/is-builtin-module/-/is-builtin-module-1.0.0.tgz",
       "integrity": "sha1-VAVy0096wxGfj3bDDLwbHgN6/74=",
       "dev": true,
       "requires": {
@@ -2113,7 +2116,7 @@
     },
     "load-json-file": {
       "version": "2.0.0",
-      "resolved": "http://registry.npmjs.org/load-json-file/-/load-json-file-2.0.0.tgz",
+      "resolved": "https://registry.npmjs.org/load-json-file/-/load-json-file-2.0.0.tgz",
       "integrity": "sha1-eUfkIUmvgNaWy/eXvKq8/h/inKg=",
       "dev": true,
       "requires": {
@@ -5705,17 +5708,17 @@
       "dev": true
     },
     "sinon": {
-      "version": "6.3.4",
-      "resolved": "https://registry.npmjs.org/sinon/-/sinon-6.3.4.tgz",
-      "integrity": "sha512-NIaR56Z1mefuRBXYrf4otqBxkWiKveX+fvqs3HzFq2b07HcgpkMgIwmQM/owNjNFAHkx0kJXW+Q0mDthiuslXw==",
+      "version": "6.3.5",
+      "resolved": "https://registry.npmjs.org/sinon/-/sinon-6.3.5.tgz",
+      "integrity": "sha512-xgoZ2gKjyVRcF08RrIQc+srnSyY1JDJtxu3Nsz07j1ffjgXoY6uPLf/qja6nDBZgzYYEovVkFryw2+KiZz11xQ==",
       "dev": true,
       "requires": {
         "@sinonjs/commons": "^1.0.2",
         "@sinonjs/formatio": "^3.0.0",
-        "@sinonjs/samsam": "^2.1.1",
+        "@sinonjs/samsam": "^2.1.2",
         "diff": "^3.5.0",
         "lodash.get": "^4.4.2",
-        "lolex": "^2.7.4",
+        "lolex": "^2.7.5",
         "nise": "^1.4.5",
         "supports-color": "^5.5.0",
         "type-detect": "^4.0.8"
@@ -6267,7 +6270,7 @@
     },
     "through": {
       "version": "2.3.8",
-      "resolved": "http://registry.npmjs.org/through/-/through-2.3.8.tgz",
+      "resolved": "https://registry.npmjs.org/through/-/through-2.3.8.tgz",
       "integrity": "sha1-DdTJ/6q8NXlgsbckEV1+Doai4fU=",
       "dev": true
     },
diff --git a/package.json b/package.json
index a1b49a1a..f0ee2253 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,7 @@
   "description": "Loads environment variables from .env file",
   "main": "lib/main.js",
   "scripts": {
+    "flow": "flow",
     "pretest": "npm run lint",
     "test": "tap tests/*.js --100",
     "lint": "standard",
@@ -26,7 +27,8 @@
   "readmeFilename": "README.md",
   "license": "BSD-2-Clause",
   "devDependencies": {
-    "sinon": "^6.3.4",
+    "flow-bin": "^0.82.0",
+    "sinon": "^6.3.5",
     "standard": "^12.0.1",
     "standard-markdown": "^5.0.1",
     "tap": "^12.0.1"
@@ -34,5 +36,10 @@
   "dependencies": {},
   "engines": {
     "node": ">=6"
+  },
+  "standard": {
+    "ignore": [
+      "flow-typed/"
+    ]
   }
 }
diff --git a/tests/test-cli-options.js b/tests/test-cli-options.js
index d9fdb748..5ecf02ff 100644
--- a/tests/test-cli-options.js
+++ b/tests/test-cli-options.js
@@ -1,8 +1,10 @@
+/* @flow */
+
 const t = require('tap')
 
 const options = require('../lib/cli-options')
 
-t.plan(4)
+t.plan(5)
 
 // matches encoding option
 t.same(options(['node', '-e', "'console.log(testing)'", 'dotenv_config_encoding=utf8']), {
@@ -14,6 +16,11 @@ t.same(options(['node', '-e', "'console.log(testing)'", 'dotenv_config_path=/cus
   path: '/custom/path/to/your/env/vars'
 })
 
+// matches debug option
+t.same(options(['node', '-e', "'console.log(testing)'", 'dotenv_config_debug=true']), {
+  debug: 'true'
+})
+
 // ignores empty values
 t.same(options(['node', '-e', "'console.log(testing)'", 'dotenv_config_path=']), {})
 
diff --git a/tests/test-config-cli.js b/tests/test-config-cli.js
index 548bef26..c5281165 100644
--- a/tests/test-config-cli.js
+++ b/tests/test-config-cli.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 const cp = require('child_process')
 const path = require('path')
 
@@ -25,5 +27,5 @@ test('config preload loads .env', t => {
     }
   )
 
-  t.equal(stdout.trim(), 'basic')
+  t.equal(stdout, 'basic\n')
 })
diff --git a/tests/test-config.js b/tests/test-config.js
index 68c4fd83..8f883c6b 100644
--- a/tests/test-config.js
+++ b/tests/test-config.js
@@ -1,22 +1,25 @@
+/* @flow */
+
 const fs = require('fs')
 
-const t = require('tap')
 const sinon = require('sinon')
+const t = require('tap')
 
 const dotenv = require('../lib/main')
 
 const mockParseResponse = { test: 'foo' }
 let readFileSyncStub
+let parseStub
 
 t.beforeEach(done => {
   readFileSyncStub = sinon.stub(fs, 'readFileSync').returns('test=foo')
-  sinon.stub(dotenv, 'parse').returns(mockParseResponse)
+  parseStub = sinon.stub(dotenv, 'parse').returns(mockParseResponse)
   done()
 })
 
 t.afterEach(done => {
   readFileSyncStub.restore()
-  dotenv.parse.restore()
+  parseStub.restore()
   done()
 })
 
@@ -38,12 +41,22 @@ t.test('takes option for encoding', ct => {
   ct.equal(readFileSyncStub.args[0][1].encoding, testEncoding)
 })
 
+t.test('takes option for debug', ct => {
+  ct.plan(1)
+
+  const logStub = sinon.stub(console, 'log')
+  dotenv.config({ debug: 'true' })
+
+  ct.ok(logStub.called)
+  logStub.restore()
+})
+
 t.test('reads path with encoding, parsing output to process.env', ct => {
   ct.plan(2)
 
   const res = dotenv.config()
-  ct.same(res.parsed, mockParseResponse)
 
+  ct.same(res.parsed, mockParseResponse)
   ct.equal(readFileSyncStub.callCount, 1)
 })
 
@@ -51,6 +64,7 @@ t.test('makes load a synonym of config', ct => {
   ct.plan(2)
 
   const env = dotenv.load()
+
   ct.same(env.parsed, mockParseResponse)
   ct.equal(readFileSyncStub.callCount, 1)
 })
@@ -63,7 +77,7 @@ t.test('does not write over keys already in process.env', ct => {
   // 'foo' returned as value in `beforeEach`. should keep this 'bar'
   const env = dotenv.config()
 
-  ct.equal(env.parsed.test, mockParseResponse.test)
+  ct.equal(env.parsed && env.parsed.test, mockParseResponse.test)
   ct.equal(process.env.test, existing)
 })
 
@@ -75,7 +89,7 @@ t.test('does not write over keys already in process.env if the key has a falsy v
   // 'foo' returned as value in `beforeEach`. should keep this ''
   const env = dotenv.config()
 
-  ct.equal(env.parsed.test, mockParseResponse.test)
+  ct.equal(env.parsed && env.parsed.test, mockParseResponse.test)
   // NB: process.env.test becomes undefined on Windows
   ct.notOk(process.env.test)
 })
diff --git a/tests/test-parse.js b/tests/test-parse.js
index 4210933c..cd517b6f 100644
--- a/tests/test-parse.js
+++ b/tests/test-parse.js
@@ -1,5 +1,8 @@
+/* @flow */
+
 const fs = require('fs')
 
+const sinon = require('sinon')
 const t = require('tap')
 
 const dotenv = require('../lib/main')
@@ -7,7 +10,7 @@ const dotenv = require('../lib/main')
 process.env.TEST = 'test'
 const parsed = dotenv.parse(fs.readFileSync('tests/.env', { encoding: 'utf8' }))
 
-t.plan(16)
+t.plan(17)
 
 t.type(parsed, Object, 'should return an object')
 
@@ -41,3 +44,9 @@ t.equal(parsed['USERNAME'], 'therealnerdybeast@example.tld', 'parses email addre
 
 const payload = dotenv.parse(Buffer.from('BASIC=basic'))
 t.equal(payload.BASIC, 'basic', 'should parse a buffer from a file into an object')
+
+// test debug path
+const logStub = sinon.stub(console, 'log')
+dotenv.parse(Buffer.from('what is this'), { debug: true })
+t.ok(logStub.called)
+logStub.restore()