Peerfinder.Start() gives System.UnauthorizedAccessException

我刚开始在WP8上玩蓝牙通信。我在这里找到了一个例子：http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj207007（v=vs.105）.aspx

但是一旦点击Peerfinder.Start（），我就会收到此错误：PhoneApp3 中发生了类型为"System.UnauthorizedAccessException"的第一次机会异常.DLL

public MainPage()
{
    InitializeComponent();
    PeerFinder.Start();
}
async private void AppToApp_Click(object sender, RoutedEventArgs e)
{
    // PeerFinder.Start() is used to advertise our presence so that peers can find us. 
    // It must always be called before FindAllPeersAsync.
    var peers = await PeerFinder.FindAllPeersAsync();
    if (peers.Count == 0)
    {
        Debug.WriteLine("Peer not found.");
    }
    else
    {
        Debug.WriteLine(peers.Count + " peers found");
    }
}