NotSupportedException in HttpWebRequest on Windows Phone 7

I have a Windows Phone 7 application built with silverlight. This application has been deployed. I've noticed in the log files that occasionally, my user's actions throw a "NotSupportedException". I have not been able to produce this. However, because of my logs, I know that it is happening in the Execute method shown here:

public void Execute()
{
  try
  {
    // 1. Build the query
   string serviceUrl = GetServiceUrl;

    // 2. Asynchronously execute the query using HttpWebRequest
    WebRequest request = HttpWebRequest.Create(serviceUrl);
    request.BeginGetResponse(new AsyncCallback(ServiceRequest_Completed), request);
  }   catch (Exception ex)
  {
    LogException(ex, "1");
  }
}


private void ServiceRequest_Completed(IAsyncResult result)
{
  try
  {
    // 1. Get the response from the service call
    WebRequest request = (WebRequest)(result.AsyncState);
    WebResponse response = request.EndGetResponse(result);

    // 2. Do stuff with response
  } 
  catch (Exception ex)
  {
    LogExcept开发者_StackOverflowion(ex, "2");
  }
}

I know it is happening in the Execute method because the "1" is written in the log file instead of the "2" My question is, what would cause this? I looked at the MSDN documentation and it looks like I'm doing what I should be doing. Like I said, I can't reproduce it locally. But I do know that it is happening regularly by different users because of the log files.

---

There is a previous question with a very similar title - https://stackoverflow.com/questions/4053197/httpwebrequest-leads-me-to-system-notsupportedexception

The answer to that problem seems to have been using ServiceRequest_Completed instead of new AsyncCallback(ServiceRequest_Completed)