无效的HTTP响应底层连接已关闭：

Question

您好我在Paypal集成中遇到一些问题，我在调试时发现此块的错误。

ReceiverList sl = new ReceiverList();
           sl.receiver.Add(rec);
           PayRequest req = new PayRequest(new RequestEnvelope("en_US"), "PAY",
                              cancelURl, "USD",
                              sl, returnURL);
           req.reverseAllParallelPaymentsOnError = true;
           AdaptivePaymentsService service = new AdaptivePaymentsService();
            PayResponse resp = null;
            resp = service.Pay(req);

Service.Pay（REQ）;抛出错误：无效的HTTP响应底层连接已关闭：发送

时发生意外错误