Opening media file with button press

Question

I have one button on my wpf called 'Open Media' with click event and a MediaElement to play media. I want to open a media file by pressing that button.

It's giving me run time exception 'System.NotSupportedException' on line 'mePlayer.Play()'. How to achieve this using following code:

    private void OpenMediaButton_Click(object sender, RoutedEventArgs e)
    {
        OpenFileDialog openMedia = new OpenFileDialog();
        openMedia.Title = "Open Media";
        openMedia.Filter = "mp4 files(*.mp4)|*.mp4";
        openMedia.InitialDirectory = @"C:\";
        openMedia.RestoreDirectory = true;
        if (openMedia.ShowDialog() == true)
        {
            mePlayer.Source = new Uri(openMedia.FileName);
            mePlayer.Position = TimeSpan.FromSeconds(1);
            mePlayer.Play();
        } 
    }