WindowsPhone MediaTranscoder with MediaStreamSource - windows-phone-8.1

Question

I'm trying to create a Video with an Image using MediaTranscoder class in WindowsPhone 8.1 SDK. If I succeed I would then use it to create a video with more images.
in the code below I find an image inside the pictures library and create a MediaStreamSample from it. this works fine!
then I create a MediaStreamSource with duration of 1 (Sec) and relate it to the sample I Created before. this works fine too!
What doesn't work is the MediaTranscoder instance.
await transcoder.PrepareMediaStreamSourceTranscodeAsync(src, ras, profile);
when I call the above method I get a "Reference not set to an instance" exception. (null Reference exception)
I don't know why this happens... please help me.
StorageFolder cameraFolder = KnownFolders.CameraRoll;
IReadOnlyList<StorageFile> fileList = await cameraFolder.GetFilesAsync();
var query = fileList.Where(f => f.Name.StartsWith("JeyLapse")).ToList();
var stream = await query[0].OpenStreamForReadAsync();
MediaStreamSample sample =
await MediaStreamSample.CreateFromStreamAsync(stream.AsInputStream(), (uint)stream.Length, new TimeSpan());
MediaStreamSource src = new MediaStreamSource(new VideoStreamDescriptor(VideoEncodingProperties.CreateMpeg2()));
src.Duration = TimeSpan.FromSeconds(1);
src.SampleRequested += (sender, args) =>
{
args.Request.Sample = sample;
};
src.Starting += (sender, args) => args.Request.SetActualStartPosition(new TimeSpan());
src.SwitchStreamsRequested += (sender, args) => args.Request.GetDeferral().Complete();
MediaTranscoder transcoder = new MediaTranscoder();
MediaEncodingProfile profile = MediaEncodingProfile.CreateMp4(VideoEncodingQuality.Auto);
var newFile = await KnownFolders.VideosLibrary.CreateFileAsync("tryJeylapse", CreationCollisionOption.GenerateUniqueName);
var writestr = await newFile.OpenStreamForWriteAsync();
var ras = writestr.AsRandomAccessStream();
transcoder.VideoProcessingAlgorithm = MediaVideoProcessingAlgorithm.Default;
var trans = await transcoder.PrepareMediaStreamSourceTranscodeAsync(src, ras, profile);
await trans.TranscodeAsync();