无法返回JSON数据,wcf resful service.net 4.0

我最近用Entity Framework 4.0建立了一个wcf resful服务 它与XML完美结合,但是当我尝试以JSON格式返回它时,我得到了

HTTP/1.1 504 Fiddler - Receive Failure
Content-Type: text/html
Connection: close
Timestamp: 01:11:06.453

ReadResponse() failed: The server did not return a response for this request.

有什么想法吗??

提前谢谢

编辑: 代码很正常,实际上我尝试了两种方法,但没有运气。

硬编码应答方式:

[OperationContract]
        [WebInvoke(Method = "GET",
            BodyStyle = WebMessageBodyStyle.Wrapped,
            ResponseFormat = WebMessageFormat.Json,
            UriTemplate = "Deal/{id}")]
        Deals XMLDealDetail(string id);

动态设置响应格式:

    [OperationContract]
            [WebInvoke(Method = "GET",
                BodyStyle = WebMessageBodyStyle.Wrapped,
                ResponseFormat = WebMessageFormat.Json,
                UriTemplate = "Deal/{id}/{format}")]
            Deals XMLDealDetail(string id, string format);

    public Deals XMLDealDetail(string id, string format)
            {
                OutgoingWebResponseContext context = WebOperationContext.Current.OutgoingResponse;

                if (format.ToLower() == "json")
                {
                    context.Format = WebMessageFormat.Json;
                    context.ContentType = "application/json";
                }
                else
                {
                    context.Format = WebMessageFormat.Xml;
                }
//Deals is a Entity Class defined in edmx file
                    Deals deal = DealsServices.GetById(id);
                    return deal;

            }

我在哪里失踪??