diff --git a/src/ajax.js b/src/ajax.js
index aec26830a2..4cbefabaaf 100644
--- a/src/ajax.js
+++ b/src/ajax.js
@@ -459,12 +459,14 @@ jQuery.extend( {
 						if ( !responseHeaders ) {
 							responseHeaders = {};
 							while ( ( match = rheaders.exec( responseHeadersString ) ) ) {
-								responseHeaders[ match[ 1 ].toLowerCase() ] = match[ 2 ];
+								responseHeaders[ match[ 1 ].toLowerCase() + " " ] =
+									( responseHeaders[ match[ 1 ].toLowerCase() + " " ] || [] )
+										.concat( match[ 2 ] );
 							}
 						}
-						match = responseHeaders[ key.toLowerCase() ];
+						match = responseHeaders[ key.toLowerCase() + " " ];
 					}
-					return match == null ? null : match;
+					return match == null ? null : match.join( ", " );
 				},
 
 				// Raw string
diff --git a/test/data/mock.php b/test/data/mock.php
index 692c417e8c..937e331bff 100644
--- a/test/data/mock.php
+++ b/test/data/mock.php
@@ -114,6 +114,9 @@ protected function headers( $req ) {
 		header( 'Sample-Header: Hello World' );
 		header( 'Empty-Header: ' );
 		header( 'Sample-Header2: Hello World 2' );
+		header( 'List-Header: Item 1' );
+		header( 'list-header: Item 2' );
+		header( 'constructor: prototype collision (constructor)' );
 
 		foreach ( explode( '|' , $req->query[ 'keys' ] ) as $key ) {
 			// Only echo if key exists in the header
diff --git a/test/middleware-mockserver.js b/test/middleware-mockserver.js
index 35623761de..09371bbe90 100644
--- a/test/middleware-mockserver.js
+++ b/test/middleware-mockserver.js
@@ -132,7 +132,10 @@ var mocks = {
 		resp.writeHead( 200, {
 			"Sample-Header": "Hello World",
 			"Empty-Header": "",
-			"Sample-Header2": "Hello World 2"
+			"Sample-Header2": "Hello World 2",
+			"List-Header": "Item 1",
+			"list-header": "Item 2",
+			"constructor": "prototype collision (constructor)"
 		} );
 		req.query.keys.split( "|" ).forEach( function( key ) {
 			if ( req.headers[ key.toLowerCase() ] ) {
diff --git a/test/unit/ajax.js b/test/unit/ajax.js
index b8b46e245c..f9cd3668d9 100644
--- a/test/unit/ajax.js
+++ b/test/unit/ajax.js
@@ -252,7 +252,7 @@ QUnit.module( "ajax", {
 		} );
 	} );
 
-	ajaxTest( "jQuery.ajax() - headers", 5, function( assert ) {
+	ajaxTest( "jQuery.ajax() - headers", 8, function( assert ) {
 		return {
 			setup: function() {
 				jQuery( document ).ajaxSend( function( evt, xhr ) {
@@ -293,6 +293,9 @@ QUnit.module( "ajax", {
 					assert.strictEqual( emptyHeader, "", "Empty header received" );
 				}
 				assert.strictEqual( xhr.getResponseHeader( "Sample-Header2" ), "Hello World 2", "Second sample header received" );
+				assert.strictEqual( xhr.getResponseHeader( "List-Header" ), "Item 1, Item 2", "List header received" );
+				assert.strictEqual( xhr.getResponseHeader( "constructor" ), "prototype collision (constructor)", "constructor header received" );
+				assert.strictEqual( xhr.getResponseHeader( "__proto__" ), null, "Undefined __proto__ header not received" );
 			}
 		};
 	} );