使用CameraCaptureTask时无法接受图像

<CaptureElement x:Name="captureElement" />

using Windows.Media.Capture;
using Windows.UI.Xaml.Controls;

private MediaCapture mediaCapture;

public async Task InitializeCameraAsync()
{
    mediaCapture = new MediaCapture();
    await mediaCapture.InitializeAsync();

    captureElement.Source = mediaCapture;
    await mediaCapture.StartPreviewAsync();
}

<Button Content="Capture" Click="CaptureButton_Click" />

private async void CaptureButton_Click(object sender, RoutedEventArgs e)
{
    var photoFile = await KnownFolders.PicturesLibrary.CreateFileAsync("photo.jpg", CreationCollisionOption.GenerateUniqueName);

    using (var captureStream = new InMemoryRandomAccessStream())
    {
        await mediaCapture.CapturePhotoToStreamAsync(ImageEncodingProperties.CreateJpeg(), captureStream);

        using (var fileStream = await photoFile.OpenAsync(FileAccessMode.ReadWrite))
        {
            var buffer = new byte[captureStream.Size];
            await captureStream.ReadAsync(buffer.AsBuffer(), (uint)captureStream.Size, InputStreamOptions.None);
            await fileStream.WriteAsync(buffer.AsBuffer());
        }
    }
}