diff --git a/addons/project_issue/project_issue.py b/addons/project_issue/project_issue.py
index a1ba0411659..b336299c3db 100644
--- a/addons/project_issue/project_issue.py
+++ b/addons/project_issue/project_issue.py
@@ -356,6 +356,7 @@ class project_issue(osv.Model):
         return res_id
 
     @api.cr_uid_ids_context
+    @api.returns('mail.message', lambda value: value.id)
     def message_post(self, cr, uid, thread_id, subtype=None, context=None, **kwargs):
         """ Overrides mail_thread message_post so that we can set the date of last action field when
             a new message is posted on the issue.