WriteableBitmap – Invalid Pointer from Constructor

　　　　Uri uri = new Uri("background.png", UriKind.Relative);
            var source = new BitmapImage();
            source.CreateOptions = BitmapCreateOptions.None;
            System.Windows.Resources.StreamResourceInfo s = Application.GetResourceStream(uri);
            source.SetSource(s.Stream);
            WriteableBitmap bmp = new WriteableBitmap(source);