Can someone help figure out why there's this t_relay error, if I decide to drop the single master branch either all branches belonging to the same destination set, even though there are more serial sets to try later?

request_route {
   seturi("sip:a@mydomain.net");
   append_branch("sip:b@mydomain.net", "0.5");
   append_branch("sip:c@mydomain.net", "0.5");
   append_branch("sip:d@10.22.0.30", "1.0");

   # append_branch("sip:e@mydomain.net", "1.0"); # no error if I add this branch with same Q=1.0

   t_load_contacts();
   t_next_contacts();

   t_on_branch("check_branch");
   t_on_failure("serial");

   t_relay();
   break;
}

branch_route[check_branch] {
   if($rd=="10.22.0.30")
      drop;
}

failure_route["serial"] {
   if (!t_next_contacts())

      exit;

}

  t_on_branch("check_branch");
  t_on_failure("serial");
  t_relay();
}

Thanks in advance.

--Sergiu