iPhone AVErrorInvalidVideoComposition ошибка при наложении изображения на видео?

Я объединил 2 фрагмента кода, найденных здесь , в один твердый фрагмент (и проверил процесс с помощью учебного файла Apple Developer Xcode). Однако когда я запускаю его, я получаю сообщение об ошибке. Он говорит:

Error Domain = AVFoundationErrorDomain Code = -11841 «Операция не может быть завершена. (AVFoundationErrorDomain error -11841.)»

Есть идеи, почему возникает ошибка AVErrorInvalidVideoComposition? Спасибо! (Я здесь новичок, дайте мне знать, если вам понадобится дополнительная информация.)

NSURL *videoURL = [info valueForKey:UIImagePickerControllerMediaURL];

/// UIImage into CALayer
UIImage *myImage = [UIImage imageNamed:@"Test.png"];
CALayer *aLayer = [CALayer layer];
aLayer.contents = (id)myImage.CGImage;

AVURLAsset* url = [AVURLAsset URLAssetWithURL:videoURL options:nil];
AVMutableComposition *videoComposition = [[AVMutableComposition alloc] init];
NSError *error;
NSFileManager *fileManager = [NSFileManager defaultManager];

AVMutableCompositionTrack *compositionVideoTrack = [videoComposition  addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
AVAssetTrack *clipVideoTrack = [[url tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [url duration])  ofTrack:clipVideoTrack atTime:kCMTimeZero error:&error];

AVMutableVideoComposition* videoComp = [[AVMutableVideoComposition alloc] init];
videoComp.renderSize = CGSizeMake(640, 480);
videoComp.frameDuration = CMTimeMake(1, 30);
CALayer *parentLayer = [CALayer layer];
CALayer *videoLayer = [CALayer layer];
parentLayer.frame = CGRectMake(0, 0, videoComp.renderSize.width, videoComp.renderSize.height);
videoLayer.frame = CGRectMake(0, 0, videoComp.renderSize.width, videoComp.renderSize.height);
[parentLayer addSublayer:videoLayer];
[parentLayer addSublayer:aLayer];
videoComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];


/// instruction
AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(60, 30) );
AVMutableVideoCompositionLayerInstruction* layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];
instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];
videoComp.instructions = [NSArray arrayWithObject: instruction];

/// outputs
NSString *filePath = nil;
filePath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
filePath = [filePath stringByAppendingPathComponent:@"temp.mov"]; 
NSLog(@"exporting to: %@", filePath);
if ([fileManager fileExistsAtPath:filePath]) 
{
    BOOL success = [fileManager removeItemAtPath:filePath error:&error];
    if (!success) NSLog(@"FM error: %@", [error localizedDescription]);
}

/// exporting
AVAssetExportSession *exporter;
exporter = [[AVAssetExportSession alloc] initWithAsset:videoComposition presetName:AVAssetExportPresetHighestQuality] ;
exporter.videoComposition = videoComp;
exporter.outputURL=[NSURL fileURLWithPath:filePath];
exporter.outputFileType=AVFileTypeQuickTimeMovie;


[exporter exportAsynchronouslyWithCompletionHandler:^(void){
    switch (exporter.status) {
        case AVAssetExportSessionStatusFailed:
            NSLog(@"exporting failed:%@",exporter.error);
            break;
        case AVAssetExportSessionStatusCompleted:
            NSLog(@"exporting completed");
            UISaveVideoAtPathToSavedPhotosAlbum(filePath, self, @selector(video:didFinishSavingWithError:contextInfo:), NULL);
            break;
        case AVAssetExportSessionStatusCancelled:
            NSLog(@"export cancelled");
            break;
    }
}];