Index: src/core.js
===================================================================
--- src/core.js	(revision 5772)
+++ src/core.js	(working copy)
@@ -615,7 +615,7 @@
 	// See test/unit/core.js for details concerning this function.
 	isFunction: function( fn ) {
 		return !!fn && typeof fn != "string" && !fn.nodeName &&
-			fn.constructor != Array && /^[\s[]?function/.test( fn + "" );
+			fn.constructor != Array && /^[\s[]?function/.test( Object.toString.call(fn) );
 	},
 
 	// check if an element is in a (or is an) XML document