WCF RIA DomainService: POST to JSON Endpoint Always Shows NULL.

Question

I've got a DomainService that I've created from an entity framework that's being exposed via a JSON endpoint. I can successfully do GET operations but whenever I try to POST to them using JSON the objects are always null.

The code below is used to create a user record in my database:

 [Invoke]
    public void CreateUser(User NewUser)
    {
        if (!DoesUserExist(NewUser.DisplayName))
        {
            ObjectContext.Users.AddObject(NewUser);
        }
    }

Which is called from my Silverlight application via this code:

DataContractJsonSerializer UserSerializer = new DataContractJsonSerializer(typeof(User));
            MemoryStream MS = new MemoryStream();
            UserSerializer.WriteObject(MS, NewUser);
            StreamReader Reader = new StreamReader(MS);
            MS.Position = 0;
            string JSON = Reader.ReadToEnd();
            WebClient CreateUser = new WebClient();

            CreateUser.UploadStringCompleted += CreateUserCompleted;
            CreateUser.Headers["Content-type"] = "application/json";
            CreateUser.Encoding = Encoding.UTF8;
            CreateUser.UploadStringAsync(new Uri("http://localhost/NewApp-web-api.svc/json/CreateUser"), "POST", JSON);

When debugging the DomainService I can see that the service gets hit however the NewUser object is always NULL. I've done some digging and seen a couple people saying they need to add a wrapper around the JSON string but no specifics about what they actually did.

Is what I'm trying to do even possible? It seems like I could accomplish this via an AJAX enabled WCF service however I lose the integration with the ObjectContext.

Thanks in advance :)