MediaElement using MediaClock

I'm trying to use a MediaElement to play a video, to which I'm
connecting a Slider as a progressbar/scrubber.

Video would work fine with the simple MediaElement controls, but now
that I'm using a MediaClock, I can't seem to get any video.

Here's what I have-- a file dialog is popped up and I want the
MediaElement to be sourced to that chosen file. Worked fine when I
used MediaElement.Source, but going through the MediaTimeline doesn't
seem to work.

private void OpenFile(object sender, EventArgs e)
{
Microsoft.Win32.OpenFileDialog ofd = new
Microsoft.Win32.OpenFileDialog();
ofd.Filter = "Video
Files|*.AVI;*.WMF;*.WMV;*.MPG;*.MPEG|All Files (*.*)|*.*";
bool? success = ofd.ShowDialog();
if (success != null && (bool) success)
{
Uri source = new Uri(@"file:///" + ofd.FileName);
MediaTimeline mt = new MediaTimeline(source);

MediaClock mc = mt.CreateClock();
videoDisplay.Clock = mc;
mc.CurrentTimeInvalidated += new
EventHandler(mc_CurrentTimeInvalidated);
mc.Controller.Begin();
}
}

Thanks!
-greg