diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 180d6667c71..c67313a477f 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -1838,7 +1838,8 @@ class MailThread(models.AbstractModel):
         """ Returns suggested recipients for ids. Those are a list of
         tuple (partner_id, partner_name, reason, default_create_value), to be managed by Chatter. """
         result = dict((res_id, []) for res_id in self.ids)
-        if 'user_id' in self._fields:
+        user_field = self._fields.get('user_id')
+        if user_field and user_field.type == 'many2one' and user_field.comodel_name == 'res.users':
             for obj in self.sudo():  # SUPERUSER because of a read on res.users that would crash otherwise
                 if not obj.user_id or not obj.user_id.partner_id:
                     continue