diff --git a/addons/calendar/calendar.py b/addons/calendar/calendar.py
index d45fd2d13e7..c76e4f0b6ec 100644
--- a/addons/calendar/calendar.py
+++ b/addons/calendar/calendar.py
@@ -1408,6 +1408,7 @@ class calendar_event(osv.Model):
             event.message_needaction = rec.message_needaction
 
     @api.cr_uid_ids_context
+    @api.returns('mail.message', lambda value: value.id)
     def message_post(self, cr, uid, thread_id, context=None, **kwargs):
         if isinstance(thread_id, basestring):
             thread_id = get_real_ids(thread_id)