web api使用JObject接收时，报“无法创建抽象类”错误

public static void RegisterRoutes(RouteCollection routes)
{
     ModelBinders.Binders.Add(typeof(JObject), new JObjectModelBinder());
}

public class JObjectModelBinder : IModelBinder
    {
        public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            var stream = controllerContext.RequestContext.HttpContext.Request.InputStream;
            stream.Seek(0, SeekOrigin.Begin);
            string json = new StreamReader(stream).ReadToEnd();
            return JsonConvert.DeserializeObject<dynamic>(json);
        }
    }