Index: categories_event_handler.php
===================================================================
--- categories_event_handler.php	(revision 14260)
+++ categories_event_handler.php	(working copy)
@@ -1595,7 +1595,7 @@
 				$object->SetError('Template', 'no_inherit');
 			}
 
-			if (!$this->Application->isAdminUser) {
+			if ( !$this->Application->isAdminUser && isset($object->VirtualFields['cust_RssSource']) ) {
 				// only administrator can set/change "cust_RssSource" field
 
 				if ($object->GetDBField('cust_RssSource') != $object->GetOriginalField('cust_RssSource')) {