XML根元素缺少问题c＃

Question

我只是将xml传递给UPS Web服务，Web服务返回一些数据。一段时间我收到错误缺少根元素这里是我的例程代码，它会抛出此错误。请指导我如何解决它。

public ShippingAcceptResult ShippingAccept(string acceptDigestCode)
{
    requestType = ShipRequestTypes.ShipAccept;

    xmlRequest = new System.Xml.XmlDocument();
    xmlRequest.LoadXml(@"<?xml version=""1.0""?>
     <ShipmentAcceptRequest>
       <Request>
          <TransactionReference>
             <CustomerContext>TR01</CustomerContext>
             <XpciVersion>1.0001</XpciVersion>
          </TransactionReference>
          <RequestAction>ShipAccept</RequestAction>
          <RequestOption>01</RequestOption>
       </Request>
       <ShipmentDigest>" + acceptDigestCode + "</ShipmentDigest></ShipmentAcceptRequest>");

    byte[] bb = new System.Text.ASCIIEncoding().GetBytes(string.Format(XML_CONNECT, sAccessCode.Trim(), sUserId.Trim(), sPassword.Trim()));

    System.IO.MemoryStream ms = new System.IO.MemoryStream();
    ms.Write(bb, 0, bb.Length);
    xmlRequest.Save(ms);
    bb = ms.ToArray();
    System.Net.WebClient wc = new System.Net.WebClient();
    xmlRespone = new XmlDocument();
    //System.Web.HttpContext.Current.Response.Write(System.Text.ASCIIEncoding.ASCII.GetString(wc.UploadData(UPS_URL, "POST", bb)));
    string serverName = (testMode) ? "wwwcie" : "www";

    serverName = string.Format(UPS_SERVICE_URL, serverName, ShipRequestTypes.ShipAccept);
    xmlRespone.LoadXml(System.Text.ASCIIEncoding.ASCII.GetString(wc.UploadData(serverName, "POST", bb)));
    //xmlRespone.Save(@"d:\shippingresponseAccept.xml");
    return new ShippingAcceptResult(xmlRespone);
}

最有趣的是如果我在例程的开头添加了thread.sleep（），那么就不会发生错误。请详细告诉我代码中存在问题的地方。感谢

修改

此行抛出错误xmlRespone.LoadXml(System.Text.ASCIIEncoding.ASCII.GetString(wc.UploadData(serverName, "POST", bb)));