WebException thrown when locking screen of the Emulator (WindowsPhone8)

I have a webrequest to get a xml.That works great but when i press F12(lock screen) while the the server is requested by my app...I got a WebException.
I use a taskCompeltionSource object...
Here is my code

public async Task<String> Query(DataRequestParam dataRequestParam)
        {

            _dataRequestParam = dataRequestParam;

            try
            {
                Result = "";
                Result = await myDownloadString(dataRequestParam);

            }

            catch (WebException we)//ERROR IS CAUGHT HERE
            {
                throw new WebException(we.Message);

            }
            catch (Exception ex)
            {
                throw new MyException(ex.Message);

            }

            return Result;
        }

        public static Task<string> myDownloadString(DataRequestParam dataRequestParam)
        {
            var tcs = new TaskCompletionSource<string>();
            var web = new WebClient();

            if (!string.IsNullOrEmpty(dataRequestParam.AuthentificationLogin))
            {
                System.Net.NetworkCredential account = new NetworkCredential(dataRequestParam.AuthentificationLogin, dataRequestParam.AuthentificationPassword);
                web.Credentials = account;
            }

                web.DownloadStringCompleted += (s, e) =>
                {
                    if (e.Error != null) tcs.TrySetException(e.Error);
                    else if (e.Cancelled) tcs.TrySetCanceled();
                    else tcs.TrySetResult(e.Result);
                };

                web.DownloadStringAsync(dataRequestParam.TargetUri);
                return tcs.Task;

        }

If you haven't disabled ApplisationIdleDetection, your process is stopped while entering Lock screen - thus you probably get the exception - like I've said in comment. Disabling will solve this issue, but you must be aware of few things:

• you will still get the exception when hitting Start Button (or other case putting your app to dormant state). In this case your app is stopped and there is no way to prevent this behaviour.
• you must fulfill certification requirements when disabling App Idle Detection - point 6.3
• if you want to download files in the Background (lock screen, after closing/leaving app) then you can think of Background Transfers