UIImagePickerController thumbnail of video which is pick up from library

Hi,:)

I am trying to get the thumbnail of the video which is pick up from library using the UIImagePickerController.

Here is my code. I can get the the thumbnail of video if I using the camera to record a video. But I can not get the thumbnail if I pick up a video from the library. Anybody know why?

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
NSString *mediaType = [info objectForKey:UIImagePickerControllerMediaType];

if([mediaType isEqualToString:(NSString *)kUTTypeMovie]){
NSURL *mediaUrl = [info objectForKey:UIImagePickerControllerMediaURL];

MPMoviePlayerController *moviePlayer = [[MPMoviePlayerController alloc] initWithContentURL:mediaUrl];
moviePlayer.shouldAutoplay = NO;
UIImage *thumbnail = [[moviePlayer thumbnailImageAtTime:0.0 timeOption:MPMovieTimeOptionNearestKeyFrame] retain];
[imageView setImage:thumbnail]; //imageView is a UIImageView

[moviePlayer release];
[thumbnail release];
[self dismissModalViewControllerAnimated:YES];
return;

}