diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 869d56bb..63711b04 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -23,11 +23,6 @@
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
-require_once(dirname(__FILE__) . '/kolab_calendar.php');
-require_once(dirname(__FILE__) . '/kolab_user_calendar.php');
-require_once(dirname(__FILE__) . '/kolab_invitation_calendar.php');
-
-
 class kolab_driver extends calendar_driver
 {
   const INVITATIONS_CALENDAR_PENDING  = '--invitation--pending';
@@ -56,6 +51,11 @@ class kolab_driver extends calendar_driver
   {
     $cal->require_plugin('libkolab');
 
+    // load helper classes *after* libkolab has been loaded (#3248)
+    require_once(dirname(__FILE__) . '/kolab_calendar.php');
+    require_once(dirname(__FILE__) . '/kolab_user_calendar.php');
+    require_once(dirname(__FILE__) . '/kolab_invitation_calendar.php');
+
     $this->cal = $cal;
     $this->rc = $cal->rc;
     $this->_read_calendars();