Index: includes/ajax.inc
===================================================================
--- includes/ajax.inc
+++ includes/ajax.inc
@@ -401,7 +401,9 @@
       );
     }
 
-    $result['#commands'] = array_merge($commands, $result['#commands']);
+    $result['#commands'] = isset($result['#commands'])
+      ? array_merge($commands, $result['#commands'])
+      : $commands;
 
     return $result;
   }