DefaultModelBinder not binding model (always null) even though ModelState contains all required values

Question

I am working with the sample ASP.NET MVC 3 application. I am working on a proof of concept for an API for a project I'm working on, by sending log in requests using HttpWebRequests. I am submitting data as JSON and am using the Content Type "application/json".

Here is the method I have:

[HttpPost]
[Api]
public bool ApiLogOn(LogOnModel model)
{
    if (ModelState.IsValid && model != null)
    {
        if (Membership.ValidateUser(model.UserName, model.Password))
        {
            FormsAuthentication.SetAuthCookie(model.UserName, model.RememberMe);
            return true;
        }
    }

    return false;
}

The problem is that even though ModelState contains all the keys and values that I expect to see (e.g. Dictionary contains { "UserName"="username", "Password"="password", "RememberMe"=true } which are the three properties of the LogOnModel class), the object model is null every time I call the method.

I do not have this problem when using "application/x-www-form-urlencoded". I have also tried with and without adding

ValueProviderFactories.Factories.Add(new JsonValueProviderFactory());

to my Application_Start method.

Here is the code I am using to send requests to the method above:

var credentials = new LogOnModel() { UserName = "username", Password = "password", RememberMe = true };
var result = PostData<LogOnModel, bool>(@"url goes here", credentials);

protected R PostData<T, R>(string uri, T postData)
{
    var request = WebRequest.Create(uri) as HttpWebRequest;
    request.Headers.Add("X-Requested-With", X_REQUESTED_WITH_HEADER);
    request.Accept = "application/json";
    request.Method = "POST";
    request.CookieContainer = cookieJar;
    request.ContentType = "application/json";

    var serializerT = new DataContractJsonSerializer(typeof(T));
    var serializerR = new DataContractJsonSerializer(typeof(R));

    if (postData != null)
    {
        using (var stream = request.GetRequestStream())
        {
            serializerT.WriteObject(stream, postData);
        }
    }

    try
    {
        var response = request.GetResponse() as HttpWebResponse;

        if (response.StatusCode == HttpStatusCode.OK)
        {
            using (var stream = response.GetResponseStream())
            {
                var result = (R)serializerR.ReadObject(stream);
                return result;
            }
        }
        else
        {
            return default(R);
        }
    } catch { return default(R); }
 }