Grapevine post request issue

I am using the Grapevine server in a console application.Calls via get work perfectly.

Now I want to pass a serialized order object to an endpoint via post request, but I get the following error message:

Sent 0 request content bytes, but Content-Length promised 538

Server code (empty body):

    [RestResource]    public class OrderService {    public OrderService() { }    [RestRoute(HttpMethod = HttpMethod.POST, PathInfo = "/distribution/saveorder")]    public IHttpContext SaveOrder(IHttpContext context) {        return context;    }}

Client code:

public void Test() {    RestClient client = new RestClient();    client.Host = "localhost";    client.Port = 8120;    RestRequest request = new RestRequest("/distribution/saveorder");    request.HttpMethod = HttpMethod.POST;    request.ContentType = ContentType.JSON;    Order order = new ();    order.Buyer.Number = "4711";    OrderItem item = new ();    item.Article.Number = "1111111";    item.Quantity = 5;    order.Items.Add (item);    item = new ();    item.Article.Number = "2222222";    item.Quantity = 20;    request.Payload = JsonSerializer.Serialize(order);    request.ContentLength = request.Payload.Length;    var response = client.Execute(request);}

The error message is pretty clear, but where / how do I enter the content length - or am I using the payload incorrectly?