diff --git a/inc/myco_objects.js b/inc/myco_objects.js
index c05b51f..74ed9bf 100644
--- a/inc/myco_objects.js
+++ b/inc/myco_objects.js
@@ -74,7 +74,7 @@ function GroupList() {
   }
 
   this.isGroup=function(el) {
-    return (el.isGroup!==undefined);
+    return (jQuery.type(el)=='object' && jQuery.type(el.isGroup)=='function' && el.isGroup());
   }
 
   this.removeGroup=function(name) {