Possible authentication problem? Loading a JSON via WebClient in Silverlight 4

I'm playing with Silverlight 4, and I when my page loads, I call

beginGet("my/people/", new OpenReadCompletedEventHandler(continueLoadStamData));

that I have defined as

private void beginGet(string endpoint, OpenReadCompletedEventHandler callback){  WebClient wc = new WebClient();  wc.Credentials = new NetworkCredential(username, password);  wc.OpenReadCompleted += callback;  wc.OpenReadAsync(new Uri(baseURL + endpoint));}

and continueLoadStamData()

void continueLoadStamData(object sender, OpenReadCompletedEventArgs e){  JsonObject root = (JsonObject)JsonObject.Load(e.Result);}

My problem is that when I get to e.Result, it throws an exception. It is the same exception I get as when I tried to use WebRequest req = ...; req.Credentials = new NetworkCredential(username, password):

{System.Reflection.TargetInvocationException: An exception occurred during the operation, making the result invalid.  Check InnerException for exception details. ---> System.Net.WebException: An exception occurred during a WebClient request. ---> System.NotImplementedException: This property is not implemented by this class.   at System.Net.WebRequest.set_Credentials(ICredentials value)   at System.Net.WebClient.GetWebRequest(Uri address)   at System.Net.WebClient.OpenReadAsync(Uri address, Object userToken)   --- End of inner exception stack trace ---   --- End of inner exception stack trace ---   at System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary()   at System.Net.OpenReadCompletedEventArgs.get_Result()   at JSONSample.MainPage.continueLoadStamData(Object sender, OpenReadCompletedEventArgs e)   at System.Net.WebClient.OnOpenReadCompleted(OpenReadCompletedEventArgs e)   at System.Net.WebClient.OpenReadOperationCompleted(Object arg)}

Do you have any idea of what's going on, how I can make sure basic authentication is implemented and get my request going?

Cheers

Nik