diff --git a/src/Declaration.js b/src/Declaration.js
index 515b17d3a37..f7ffb61b2fe 100644
--- a/src/Declaration.js
+++ b/src/Declaration.js
@@ -1,5 +1,5 @@
 import { blank, forOwn, keys } from './utils/object.js';
-import makeLegalIdentifier, { reservedWords } from './utils/makeLegalIdentifier.js';
+import { makeLegal, reservedWords } from './utils/identifier-helpers.js';
 import { UNKNOWN } from './ast/values.js';
 
 export default class Declaration {
@@ -25,7 +25,7 @@ export default class Declaration {
 		reference.declaration = this;
 
 		if ( reference.name !== this.name ) {
-			this.name = makeLegalIdentifier( reference.name ); // TODO handle differences of opinion
+			this.name = makeLegal( reference.name ); // TODO handle differences of opinion
 		}
 
 		if ( reference.isReassignment ) this.isReassigned = true;
diff --git a/src/ExternalModule.js b/src/ExternalModule.js
index 469d708be51..5538bb25286 100644
--- a/src/ExternalModule.js
+++ b/src/ExternalModule.js
@@ -1,5 +1,5 @@
 import { blank } from './utils/object.js';
-import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
+import { makeLegal } from './utils/identifier-helpers.js';
 import { ExternalDeclaration } from './Declaration.js';
 
 export default class ExternalModule {
@@ -7,7 +7,7 @@ export default class ExternalModule {
 		this.id = id;
 		this.path = relativePath;
 
-		this.name = makeLegalIdentifier( relativePath );
+		this.name = makeLegal( relativePath );
 
 		this.nameSuggestions = blank();
 		this.mostCommonSuggestion = 0;
diff --git a/src/Module.js b/src/Module.js
index a1d5ec9f8f6..b533c900667 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -4,7 +4,7 @@ import { locate } from 'locate-character';
 import { timeStart, timeEnd } from './utils/flushTime.js';
 import { assign, blank, keys } from './utils/object.js';
 import { basename, extname } from './utils/path.js';
-import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
+import { makeLegal } from './utils/identifier-helpers.js';
 import getCodeFrame from './utils/getCodeFrame.js';
 import { SOURCEMAPPING_URL_RE } from './utils/sourceMappingURL.js';
 import error from './utils/error.js';
@@ -251,7 +251,7 @@ export default class Module {
 		const base = basename( this.id );
 		const ext = extname( this.id );
 
-		return makeLegalIdentifier( ext ? base.slice( 0, -ext.length ) : base );
+		return makeLegal( ext ? base.slice( 0, -ext.length ) : base );
 	}
 
 	bindImportSpecifiers () {
diff --git a/src/finalisers/iife.js b/src/finalisers/iife.js
index 239cdb5b7e2..f39d00f5855 100644
--- a/src/finalisers/iife.js
+++ b/src/finalisers/iife.js
@@ -7,6 +7,7 @@ import getGlobalNameMaker from './shared/getGlobalNameMaker.js';
 import { property, keypath } from './shared/sanitize.js';
 import warnOnBuiltins from './shared/warnOnBuiltins.js';
 import trimEmptyImports from './shared/trimEmptyImports.js';
+import { isLegal } from '../utils/identifier-helpers.js';
 
 function setupNamespace ( keypath ) {
 	const parts = keypath.split( '.' );
@@ -26,6 +27,14 @@ export default function iife ( bundle, magicString, { exportMode, indentString,
 	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle, 'null' );
 
 	const name = options.moduleName;
+
+	if ( name && !isLegal(name) ) {
+		error({
+			code: 'ILLEGAL_IDENTIFIER_AS_NAME',
+			message: `Given moduleName - ${ name } - is not legal JS identifier.`
+		})
+	}
+
 	const isNamespaced = name && ~name.indexOf( '.' );
 
 	warnOnBuiltins( bundle );
@@ -41,9 +50,12 @@ export default function iife ( bundle, magicString, { exportMode, indentString,
 		});
 	}
 
-	if ( exportMode === 'named' ) {
+	if ( isNamespaced ) {
 		dependencies.unshift( `(${thisProp(name)} = ${thisProp(name)} || {})` );
 		args.unshift( 'exports' );
+	} else if ( exportMode === 'named' ) {
+		dependencies.unshift( '{}' );
+		args.unshift( 'exports' );
 	}
 
 	const useStrict = options.useStrict !== false ? `${indentString}'use strict';\n\n` : ``;
@@ -51,7 +63,7 @@ export default function iife ( bundle, magicString, { exportMode, indentString,
 	let wrapperIntro = `(function (${args}) {\n${useStrict}`;
 	const wrapperOutro = `\n\n}(${dependencies}));`;
 
-	if ( exportMode === 'default' ) {
+	if ( exportMode !== 'none' ) {
 		wrapperIntro = ( isNamespaced ? thisProp(name) : `${bundle.varOrConst} ${name}` ) + ` = ${wrapperIntro}`;
 	}
 
diff --git a/src/utils/makeLegalIdentifier.js b/src/utils/identifier-helpers.js
similarity index 72%
rename from src/utils/makeLegalIdentifier.js
rename to src/utils/identifier-helpers.js
index 846c42e158f..654f0172047 100644
--- a/src/utils/makeLegalIdentifier.js
+++ b/src/utils/identifier-helpers.js
@@ -6,13 +6,26 @@ const builtins = 'Infinity NaN undefined null true false eval uneval isFinite is
 const blacklisted = blank();
 reservedWords.concat( builtins ).forEach( word => blacklisted[ word ] = true );
 
+const illegalCharacters = /[^$_a-zA-Z0-9]/g;
 
-export default function makeLegalIdentifier ( str ) {
+const startsWithDigit = str => /\d/.test( str[0] );
+
+export function isLegal ( str ) {
+	if ( startsWithDigit(str) || blacklisted[ str ] ) {
+		return false;
+	}
+	if ( illegalCharacters.test(str) ) {
+		return false;
+	}
+	return true;
+}
+
+export function makeLegal ( str ) {
 	str = str
 		.replace( /-(\w)/g, ( _, letter ) => letter.toUpperCase() )
-		.replace( /[^$_a-zA-Z0-9]/g, '_' );
+		.replace( illegalCharacters, '_' );
 
-	if ( /\d/.test( str[0] ) || blacklisted[ str ] ) str = `_${str}`;
+	if ( startsWithDigit(str) || blacklisted[ str ] ) str = `_${str}`;
 
 	return str;
 }