diff --git a/ui/jquery.ui.widget.js b/ui/jquery.ui.widget.js
index 59fd81b9e00..05487f7edb8 100644
--- a/ui/jquery.ui.widget.js
+++ b/ui/jquery.ui.widget.js
@@ -142,9 +142,14 @@ $.widget.extend = function( target ) {
 	for ( ; inputIndex < inputLength; inputIndex++ ) {
 		for ( key in input[ inputIndex ] ) {
 			value = input[ inputIndex ][ key ];
-			if (input[ inputIndex ].hasOwnProperty( key ) && value !== undefined ) {
-				if ( $.isPlainObject( value ) && $.isPlainObject( target[ key ] ) ) {
-					target[ key ] = $.widget.extend( {}, target[ key ], value );
+			if ( input[ inputIndex ].hasOwnProperty( key ) && value !== undefined ) {
+				// Clone objects
+				if ( $.isPlainObject( value ) ) {
+					target[ key ] = $.isPlainObject( target[ key ] ) ?
+						$.widget.extend( {}, target[ key ], value ) :
+						// Don't extend strings, arrays, etc. with objects
+						$.widget.extend( {}, value );
+				// Copy everything else by reference
 				} else {
 					target[ key ] = value;
 				}