diff --git a/app/controllers/calendar/schedule.php b/app/controllers/calendar/schedule.php
index 8202d16c2eb639357948d3150f9ee335556ee8cf..c20fbea88c8a12bf7bd695642d9c994647faf7e4 100644
--- a/app/controllers/calendar/schedule.php
+++ b/app/controllers/calendar/schedule.php
@@ -15,6 +15,10 @@
  * @category    Stud.IP
  * @since      2.0
  */
+
+// Needs to be required due to the use of constants
+require_once 'lib/classes/calendar/CalendarScheduleModel.php';
+
 class Calendar_ScheduleController extends AuthenticatedController
 {