diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 15b3cdee6df2795cba1d1dd241020d2ffde53c93..d383c8096ea985f8d7c6b60d2a8bf6ca4f80fc9e 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -180,7 +180,7 @@ class MailThread(models.AbstractModel):
                              WHERE msg.model = %s AND msg.res_id in %s AND
                                    (msg.author_id IS NULL OR msg.author_id != %s) AND
                                    (msg.message_type != 'notification' OR msg.model != 'mail.channel')""",
-                         (partner_id, self._name, tuple(self.ids), partner_id,))
+                         (partner_id, self._name, tuple(self.ids) or (None,), partner_id,))
         for result in self._cr.fetchall():
             res[result[0]] += 1