diff --git a/addons/base_calendar/base_calendar.py b/addons/base_calendar/base_calendar.py
index 850691d329c..9af977f8b8e 100644
--- a/addons/base_calendar/base_calendar.py
+++ b/addons/base_calendar/base_calendar.py
@@ -1415,6 +1415,10 @@ e.g.: Every other month on the last Sunday of the month for 10 occurrences:\
         @param context: A standard dictionary for contextual values
         @return: True
         """
+        old_vals = self.read(cr, uid, ids, 'rrule', context)[0]
+        if 'rrule' in vals and not vals['rrule'] and not old_vals['rrule']: 
+            del vals['rrule']
+        
         if context is None:
             context = {}
         if isinstance(ids, (str, int, long)):