Сериализация XML и XMLChoiceAttribute

Пытаюсь сериализовать простой объект:

public class ChargesConditions
  {
      public string supplierBillID;
      public ChargesConditions() { }
  }
  public class PayersConditions
  {
      public string payerIdentifier;
      public PayersConditions() { }
  }
  [XmlType(IncludeInSchema = false)]
  public enum ItemChoiceType
  {
      Payers,
      Charges
  }
  public class Choices
  {
      [XmlChoiceIdentifier("ItemType")]
      [XmlElement("PayersConditions", Type = typeof(PayersConditions))]
      [XmlElement("ChargesConditions", Type = typeof(ChargesConditions))]
      public object Choice;
      [XmlIgnore]
      public ItemChoiceType ItemType;
  }
  class Program
  {
      static void Main(string[] args)
      {
          ChargesConditions charge = new ChargesConditions();
          charge.supplierBillID = "123";
          Choices c1 = new Choices();
          c1.ItemType = ItemChoiceType.Charges;
          c1.Choice = charge;
          var serializer = new XmlSerializer(typeof(Choices));
          using (var stream = new FileStream("Choices.xml", FileMode.Create))
              serializer.Serialize(stream, c1);
      }
  }

Программа генерирует исключение InvalidOperationException в строке

var serializer = new XmlSerializer(typeof(Choices));

Что я делаю неправильно?