diff --git a/addons/mail/mail_message.py b/addons/mail/mail_message.py
index 84f0062cf5d..3517c1a47a3 100644
--- a/addons/mail/mail_message.py
+++ b/addons/mail/mail_message.py
@@ -977,7 +977,7 @@ class mail_message(osv.Model):
                     ('res_id', '=', message.res_id),
                 ], context=context)
             partners_to_notify |= set(
-                fo.partner_id for fo in fol_obj.browse(cr, SUPERUSER_ID, fol_ids, context=context)
+                fo.partner_id.id for fo in fol_obj.browse(cr, SUPERUSER_ID, fol_ids, context=context)
                 if message.subtype_id.id in [st.id for st in fo.subtype_ids]
             )
         # remove me from notified partners, unless the message is written on my own wall