WebService REST request POST error

When ordering from POST, I get this error message:

 O servidor remoto devolveu um erro: (400) Pedido incorrecto.

IService is defined as follows:

[WebInvoke(Method = "POST", UriTemplate = "insert/?id={id}&v={v}&dt={dt}&qt={qt}",
            RequestFormat = WebMessageFormat.Xml,
            ResponseFormat = WebMessageFormat.Xml)]
        [OperationContract]
        bool Insert(int id,int v, string dt, int qt);

and the Request I do as follows:

string uploadUrl = @"http://url/ServicePedido.svc/insert/?id=" + id + "&v=" + vim + "&dt=" + daregisto + "&qt=" + quantidade;
                WebRequest addRequest = WebRequest.Create(uploadUrl);
                addRequest.Method = "POST";
                addRequest.ContentLength = 0;
                WebResponse addResponse = addRequest.GetResponse();