MPMoviePlayerController gives me a black empty view,no video playing

iOS 5.1 ,here is the code:

 MPMoviePlayerController *player =
    [[MPMoviePlayerController alloc] initWithContentURL: [NSURL URLWithString:@"map.mp4"]];
    [player prepareToPlay];
    [player.view setFrame: self.view.bounds];  // player's frame must match parent's
    [self.view addSubview: player.view];
    // ...
    [player play];

in fact ,this is just the same as the apple reference say,but when I click the button to use this function,there is just a black Rectangle stay there,no vedio playing ,no sound happening,and no crush,so I want to know how to make this work