Index: selector.js
===================================================================
--- selector.js	(revision 5845)
+++ selector.js	(working copy)
@@ -393,6 +393,9 @@
 				var fn = jQuery.expr[ m[1] ];
 				if ( typeof fn == "object" )
 					fn = fn[ m[2] ];
+					
+				if( !fn )
+					throw m[0] + ' is not a valid filter';
 
 				if ( typeof fn == "string" )
 					fn = eval("false||function(a,i){return " + fn + ";}");