diff --git a/calendar/inc/class.calendar_uiforms.inc.php b/calendar/inc/class.calendar_uiforms.inc.php
index 659b2a4482..d267424ad1 100644
--- a/calendar/inc/class.calendar_uiforms.inc.php
+++ b/calendar/inc/class.calendar_uiforms.inc.php
@@ -3247,7 +3247,7 @@ class calendar_uiforms extends calendar_ui
 		else
 		{
 			// Group membership
-			foreach(array_keys($event['participants']) as $id)
+			foreach(array_keys($event['participants'] ?? []) as $id)
 			{
 				if($GLOBALS['egw']->accounts->get_type($id) == 'g' && in_array($uid,$GLOBALS['egw']->accounts->members($id,true)))
 				{