diff --git a/calendar/inc/class.calendar_so.inc.php b/calendar/inc/class.calendar_so.inc.php
index 67ac746d3a..ded116502b 100644
--- a/calendar/inc/class.calendar_so.inc.php
+++ b/calendar/inc/class.calendar_so.inc.php
@@ -777,7 +777,7 @@ class calendar_so
 			$private_filter = '(cal_public=1 OR cal_public=0 AND '.$this->db->expression($this->cal_table, array('cal_owner' => $params['private_grants'])) . ')';
 			$where[] = $private_filter;
 		}
-		if($params['sql_filter']['cal_id'])
+		if(isset($params['sql_filter']['cal_id']))
 		{
 			$params['query']['cal_id'] = $params['sql_filter']['cal_id'];
 			$where[] = $this->db->column_data_implode(", ", [$this->cal_table.'.cal_id' => $params['sql_filter']['cal_id']], True, False);