Index: core.js
===================================================================
--- core.js	(revision 5470)
+++ core.js	(working copy)
@@ -146,8 +146,11 @@
 
 		// Locate the position of the desired element
 		this.each(function(i){
-			if ( this == elem )
+			// use === because on IE, window == document
+			if( elem.jquery ? elem.index(this) != -1 : this === elem ){
 				ret = i;
+				return false;
+			}
 		});
 
 		return ret;