diff --git a/calendar/inc/class.calendar_boupdate.inc.php b/calendar/inc/class.calendar_boupdate.inc.php
index f2acfbe1cd..e59db1610f 100644
--- a/calendar/inc/class.calendar_boupdate.inc.php
+++ b/calendar/inc/class.calendar_boupdate.inc.php
@@ -959,7 +959,7 @@ class calendar_boupdate extends calendar_bo
 				}
 				$userid = $res_info['responsible'];
 
-				if (!isset($userid))
+				if (empty($userid))	// no resource responsible: $userid===0
 				{
 					if (empty($res_info['email'])) continue;	// no way to notify
 					// check if event-owner wants non-EGroupware users notified