diff --git a/addons/calendar/calendar.py b/addons/calendar/calendar.py
index 46d0fbb38c3..08464fe1b97 100644
--- a/addons/calendar/calendar.py
+++ b/addons/calendar/calendar.py
@@ -961,9 +961,9 @@ class calendar_event(osv.Model):
 
     def _check_closing_date(self, cr, uid, ids, context=None):
         for event in self.browse(cr, uid, ids, context=context):
-            if event.start_datetime and event.stop_datetime < event.start_datetime:
+            if event.start_datetime and event.stop_datetime and event.stop_datetime < event.start_datetime:
                 return False
-            if event.start_date and event.stop_date < event.start_date:
+            if event.start_date and event.stop_date and event.stop_date < event.start_date:
                 return False
         return True