Deserialize unknown type with protobuf-net

I have 2 networked apps that should send serialized protobuf-net messages to each other. I can serialize the objects and send them, however, I cannot figure out how to deserialize the received bytes.

I tried to deserialize with this and it failed with a NullReferenceException.

// Where "ms" is a memorystream containing the serialized
// byte array from the network.
Messages.BaseMessage message =
  ProtoBuf.Serializer.Deserialize<Messages.BaseMessage>(ms);

I am passing a header before the serialized bytes that contains message type ID, which I can use in a giant switch statement to return the expected sublcass Type. With the block below, I receive the error: System.Reflection.TargetInvocationException ---> System.NullReferenceException.

//Where "ms" is a memorystream and "messageType" is a
//Uint16.
Type t = Messages.Helper.GetMessageType(messageType);
System.Reflection.MethodInfo method =
  typeof(ProtoBuf.Serializer).GetMethod("Deserialize").MakeGenericMethod(t);
message = method.Invoke(null, new object[] { ms }) as Messages.BaseMessage;

Here's the function I use to send a message over the network:

internal void Send(Messages.BaseMessage message){
  using (System.IO.MemoryStream ms = new System.IO.MemoryStream()){
    ProtoBuf.Serializer.Serialize(ms, message);
    byte[] messageTypeAndLength = new byte[4];
    Buffer.BlockCopy(BitConverter.GetBytes(message.messageType), 0, messageTypeAndLength, 0, 2);
    Buffer.BlockCopy(BitConverter.GetBytes((UInt16)ms.Length), 0, messageTypeAndLength, 2, 2);
    this.networkStream.Write(messageTypeAndLength);
    this.networkStream.Write(ms.ToArray());
  }
}

This the class, with base class, I'm serializing:

[Serializable,
ProtoContract,
ProtoInclude(50, typeof(BeginRequest))]
abstract internal class BaseMessage
{
  [ProtoMember(1)]
  abstract public UInt16 messageType { get; }
}

[Seriali