问为什么以下的protobuf-net使用是非法的？

EN

  public interface IYObject
  {
    string X { get; }
  }

  public class YObject : IYObject
  {
    public string X { get; set; }
  }

  public class D
  {
    public IYObject Y { get; set; }
  }

  class Program
  {
    static void Main()
    {
      var m = RuntimeTypeModel.Default;
      m.Add(typeof(D), true).Add("Y");
      m.Add(typeof(IYObject), false).AddSubType(1, typeof(YObject)).Add("X");
      var d = new D { Y = new YObject { X = "a" } };
      using (var ms = new MemoryStream())
      {
        Serializer.Serialize(ms, d);
        ms.Position = 0;
        var d2 = Serializer.Deserialize<D>(ms);
        Debug.Assert(d.Y.X == d2.Y.X);
      }
    }
  }

System.InvalidOperationException occurred
  Message=Sub-types can only be adedd to non-sealed classes
  Source=protobuf-net
  StackTrace:
       at ProtoBuf.Meta.MetaType.AddSubType(Int32 fieldNumber, Type derivedType)
  InnerException: