diff --git a/calendar/inc/class.calendar_uiforms.inc.php b/calendar/inc/class.calendar_uiforms.inc.php
index 015c5be07a..bedef318c8 100644
--- a/calendar/inc/class.calendar_uiforms.inc.php
+++ b/calendar/inc/class.calendar_uiforms.inc.php
@@ -1007,7 +1007,7 @@ class calendar_uiforms extends calendar_ui
 				$response = Api\Json\Response::get();
 				if($response && $update_type != 'delete' && !$client_updated)
 				{
-					$client_updated = $this->update_client($event['id'], null, $old_event);
+					$client_updated = $this->update_client($event['id'], null, is_array($old_event) ? $old_event : []);
 				}
 
 				$msg = $message . ($msg ? ', ' . $msg : '');