@Override
  public Set<AlarmReceiver> getReceivers(Transport transport) {
    Set<AlarmReceiver> r = Sets.newHashSet();

    for (AlarmReceiver receiver : receivers) {
      String address = receiver.getAddress(transport);
      if (address != null && !address.isEmpty()) {
        r.add(receiver);
      }
    }

    return r;
  }