if (nat_uac_test(64)) {
          force_rport();
      ......
        if (!add_contact_alias()) {
            xinfo("Error aliasing contact <$ct>\n");
            sl_send_reply("400", "Bad Request");
            exit;
           }

set_contact_alias() throws this error on ACK without Contact, add_contact_alias() doesn't (and thats OK). Is this the expected behavior?