diff --git a/addons/base_calendar/base_calendar.py b/addons/base_calendar/base_calendar.py
index c289100d3ae..2cf2ad96226 100644
--- a/addons/base_calendar/base_calendar.py
+++ b/addons/base_calendar/base_calendar.py
@@ -452,11 +452,9 @@ request was delegated to"),
             sign = att.sent_by_uid and att.sent_by_uid.signature or ''
             sign = '<br>'.join(sign and sign.split('\n') or [])
             res_obj = att.ref
-            if res_obj and len(res_obj):
-                res_obj = res_obj[0]
             sub = '[%s Invitation][%d] %s'  % (company, att.id, res_obj.name)
             att_infos = []
-            other_invitaion_ids = self.search(cr, uid, [('ref','=',att.ref)])
+            other_invitaion_ids = self.search(cr, uid, [('ref','=', att.ref._name + ',' + str(att.ref.id))])
             for att2 in self.browse(cr, uid, other_invitaion_ids):
                 att_infos.append(((att2.user_id and att2.user_id.name) or \
                              (att2.partner_id and att2.partner_id.name) or \