diff --git a/calendar/inc/class.calendar_bo.inc.php b/calendar/inc/class.calendar_bo.inc.php
index 1f1153ddb0..8bf2b23d4b 100644
--- a/calendar/inc/class.calendar_bo.inc.php
+++ b/calendar/inc/class.calendar_bo.inc.php
@@ -25,6 +25,11 @@ define('WEEK_s',7*DAY_s);
  */
 define('EGW_ACL_READ_FOR_PARTICIPANTS',EGW_ACL_CUSTOM_1);
 
+/**
+ * Required (!) include, as we use the MCAL_* constants, BEFORE instanciating (and therefore autoloading) the class
+ */
+require_once(EGW_INCLUDE_ROOT.'/calendar/inc/class.calendar_so.inc.php');
+
 /**
  * Class to access all calendar data
  *