diff --git a/ui/jquery.effects.core.js b/ui/jquery.effects.core.js
index 699b41d81..3dc0a4e67 100644
--- a/ui/jquery.effects.core.js
+++ b/ui/jquery.effects.core.js
@@ -223,7 +223,7 @@ $.effects.animateClass = function( value, duration, easing, callback ) {
 
 	return this.queue( function() {
 		var animated = $( this ),
-			baseClass = animated.attr( "class" ),
+			baseClass = animated.attr( "class" ) || "",
 			finalClass,
 			allAnimations = o.children ? animated.find( "*" ).andSelf() : animated;