diff --git a/addons/base_calendar/base_calendar.py b/addons/base_calendar/base_calendar.py
index e5cff943f2f..f8ac4c46885 100644
--- a/addons/base_calendar/base_calendar.py
+++ b/addons/base_calendar/base_calendar.py
@@ -1473,7 +1473,7 @@ e.g.: Every other month on the last Sunday of the month for 10 occurrences:\
             if not real_event_id in new_ids:
                 new_ids.append(real_event_id)
 
-        if vals.get('vtimezone', '').startswith('/freeassociation.sourceforge.net/tzfile/'):
+        if vals.get('vtimezone', '') and vals.get('vtimezone', '').startswith('/freeassociation.sourceforge.net/tzfile/'):
             vals['vtimezone'] = vals['vtimezone'][40:]
 
         updated_vals = self.onchange_dates(cr, uid, new_ids,