diff --git a/addons/mass_mailing/models/mass_mailing.py b/addons/mass_mailing/models/mass_mailing.py
index f7c6a1d5127..24870ac8408 100644
--- a/addons/mass_mailing/models/mass_mailing.py
+++ b/addons/mass_mailing/models/mass_mailing.py
@@ -622,7 +622,7 @@ class MassMailing(osv.Model):
         })
         return super(MassMailing, self).copy_data(cr, uid, id, default, context=context)
 
-    def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, context=None, orderby=False):
+    def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, context=None, orderby=False, lazy=True):
         """ Override read_group to always display all states. """
         if groupby and groupby[0] == "state":
             # Default result structure