Index: src/core.js
===================================================================
--- src/core.js	(revision 5872)
+++ src/core.js	(working copy)
@@ -614,8 +614,10 @@
 	// See test/unit/core.js for details concerning this function.
 	// Since 1.3 DOM methods and function like alert
 	// aren't supported. They return false on IE (#2968).
+	// Memory leaks appear in IE6 when applyin instance of to
+	// window, document or any other COM object
 	isFunction: function( fn ) {
-		return fn instanceof Function;
+		return !!fn && !!fn.hasOwnProperty && fn instanceof Function;
 	},
 
 	// check if an element is in a (or is an) XML document