diff --git a/lib/reply.js b/lib/reply.js
index 533bad81ac..acc60826c3 100644
--- a/lib/reply.js
+++ b/lib/reply.js
@@ -129,7 +129,14 @@ Reply.prototype.header = function (key, value) {
 
   if (this._headers[_key] && _key === 'set-cookie') {
     // https://tools.ietf.org/html/rfc7230#section-3.2.2
-    this._headers[_key] = [this._headers[_key]].concat(value)
+    if (typeof this._headers[_key] === 'string') {
+      this._headers[_key] = [this._headers[_key]]
+    }
+    if (Array.isArray(value)) {
+      Array.prototype.push.apply(this._headers[_key], value)
+    } else {
+      this._headers[_key].push(value)
+    }
   } else {
     this._headers[_key] = value
   }
diff --git a/test/internals/reply.test.js b/test/internals/reply.test.js
index 8e8ea8e52f..f429444d68 100644
--- a/test/internals/reply.test.js
+++ b/test/internals/reply.test.js
@@ -916,3 +916,38 @@ test('.status() is an alias for .code()', t => {
     t.is(res.statusCode, 418)
   })
 })
+
+test('reply.header setting multiple cookies as multiple Set-Cookie headers', t => {
+  t.plan(7)
+
+  const fastify = require('../../')()
+
+  fastify.get('/headers', function (req, reply) {
+    reply
+      .header('set-cookie', 'one')
+      .header('set-cookie', 'two')
+      .header('set-cookie', 'three')
+      .header('set-cookie', ['four', 'five', 'six'])
+      .send({})
+  })
+
+  fastify.listen(0, err => {
+    t.error(err)
+    fastify.server.unref()
+
+    sget({
+      method: 'GET',
+      url: 'http://localhost:' + fastify.server.address().port + '/headers'
+    }, (err, response, body) => {
+      t.error(err)
+      t.ok(response.headers['set-cookie'])
+      t.strictDeepEqual(response.headers['set-cookie'], ['one', 'two', 'three', 'four', 'five', 'six'])
+    })
+  })
+
+  fastify.inject('/headers', (error, response) => {
+    t.error(error)
+    t.ok(response.headers['set-cookie'])
+    t.strictDeepEqual(response.headers['set-cookie'], ['one', 'two', 'three', 'four', 'five', 'six'])
+  })
+})