diff --git a/calendar/inc/class.calendar_so.inc.php b/calendar/inc/class.calendar_so.inc.php
index 1330459c9b..e3c7479410 100644
--- a/calendar/inc/class.calendar_so.inc.php
+++ b/calendar/inc/class.calendar_so.inc.php
@@ -422,7 +422,7 @@ class calendar_so
 			// changed the original OR in the query into a union, to speed up the query execution under MySQL 5
 			$select = array(
 				'table' => $this->cal_table,
-				'join'  => "JOIN $this->dates_table ON $this->cal_table.cal_id=$this->dates_table.cal_id JOIN $this->user_table ON $this->cal_table.cal_id=$this->user_table.cal_id LEFT JOIN $this->repeats_table ON $this->cal_table.cal_id=$this->repeats_table.cal_id JOIN egw_api_content_history ON egw_api_content_history.sync_appname = 'calendar' AND egw_api_content_history.sync_contentid=$this->cal_table.cal_id",
+				'join'  => "JOIN $this->dates_table ON $this->cal_table.cal_id=$this->dates_table.cal_id JOIN $this->user_table ON $this->cal_table.cal_id=$this->user_table.cal_id LEFT JOIN $this->repeats_table ON $this->cal_table.cal_id=$this->repeats_table.cal_id JOIN egw_api_content_history ON egw_api_content_history.sync_appname = 'calendar' AND CAST(egw_api_content_history.sync_contentid AS UNSIGNED)=$this->cal_table.cal_id",
 				'cols'  => $cols,
 				'where' => $where,
 				'app'   => 'calendar',