create TypeModel at runtime using Protobuf-Net, Unexpected sub-type

I have the class structure below and would like to serialize it at runtime using Protobuf-Net. Unfortunately I get error "Unexpected sub-type: Web2Pdf". Why?

var web2PdfEntity = new Web2Pdf();
web2PdfEntity.Property1 = 1; 
web2PdfEntity.Property2 = 2;
    web2PdfEntity.Property3 = 3;

var model = TypeModel.Create();
model.Add(typeof (EntityBase), true).AddSubType(20000, typeof (WebEntity)).AddSubType(30000,typeof (Web2Pdf));                
model.CompileInPlace();



  using (var stream = new FileStream(@"C:\1.txt", FileMode.Create, FileAccess.Write, FileShar开发者_开发技巧e.None))
{
   model.Serialize(stream, web2PdfEntity); //Get exception here!
}


[ProtoContract]
public abstract class EntityBase
{
   [ProtoMember(1011)]
   public int Property1 { get; set; }
}

[ProtoContract]
public abstract class WebEntity : EntityBase
{
   [ProtoMember(1012)]
   public int Property2 { get; set; }
}

[ProtoContract]
public sealed class Web2Pdf : WebEntity
{
   [ProtoMember(1013)]
   public int Property3 { get; set; }
}

The subtypes must be associated with the immediate parent, so: EntityBase needs to know about WebEntity, and WebEntity needs to know about Web2Pdf (rather than EntityBase knowing about both and WebEntity not knowing about Web2Pdf).

For info, smaller tag numbers are more efficient, too - but up to you.

Additionally, this can all be done via [ProtoInclude(...)], which can be more convenient if the sub-type numbers are fixed.