diff --git a/partner_firstname/res_user.py b/partner_firstname/res_user.py
index 490bc16ca..18d2d5930 100644
--- a/partner_firstname/res_user.py
+++ b/partner_firstname/res_user.py
@@ -36,13 +36,14 @@ class ResUsers(orm.Model):
         """
         vals2 = vals.copy()
 
-        res_partner = self.pool.get('res.partner')
-        partner = res_partner.browse(cr, user, vals2['partner_id'], context)
-
         if 'name' in vals:
             vals2['lastname'] = vals2['name']
-        elif partner.lastname and 'lastname' not in vals:
-            vals2['lastname'] = partner.lastname
+        elif 'lastname' not in vals and 'partner_id' in vals:
+            res_partner = self.pool.get('res.partner')
+            partner = res_partner.browse(cr, user, vals2['partner_id'], 
+                                         context)
+            if partner.lastname:
+                vals2['lastname'] = partner.lastname
         elif 'login' in vals and 'lastname' not in vals:
             vals2['lastname'] = vals2['login']
         return super(ResUsers, self).create(cr, user, vals2, context=context)