@Override
  public void addUnitToPerson(ProposalPerson person, ProposalPersonUnit unit) {
    if (unit == null) {
      throw new IllegalArgumentException("Cannot add null units to a ProposalPerson instance");
    }

    if (!person.containsUnit(unit.getUnitNumber())) {
      unit.setProposalPerson(person);

      person.addUnit(unit);
      unit.refreshReferenceObject("unit");
    }
  }