iOS 5：使用AVAssetExportSession合并3个视频时出错

Error Domain=AVFoundationErrorDomain Code=-11820 "CAnnot Complete Export" UserInfo=0x458120 {NSLocalizedRecoverySuggestion=Try exporTing again.,NSLocalizedDescription=CAnnot Complete Export}

我甚至尝试重做函数以防出错,但我得到的只是无限错误信息.这是我的代码的片段.

AVMutableComposition *mixComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
NSError * error = nil;
NSMutableArray * timeRanges = [NSMutableArray arrayWithCapacity:arraymovieUrl.count];
NSMutableArray * tracks = [NSMutableArray arrayWithCapacity:arraymovieUrl.count];

for (int i=0; i<[arraymovieUrl count]; i++) {
    AVURLAsset *assetClip = [arraymovieUrl objectATindex:i];
    AVAssetTrack *clipVideoTrackB = [[assetClip tracksWithMediaType:AVMediaTypeVideo] objectATindex:0];

    [timeRanges addObject:[NSValue valueWithCMTimeRange:CMTimeRangeMake(kCMTimeZero,assetClip.duration)]];
    [tracks addObject:clipVideoTrackB];
}
[compositionTrack insertTimeRanges:timeRanges ofTracks:tracks atTime:kCMTimeZero error:&error];

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPreset1280x720];
NSParameterAssert(exporter != nil);
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.outputURL = outputUrl;
[exporter exportAsynchronouslyWithCompletionHandler:^{
    switch ([exporter status]) {
        case AVAssetExportSessionStatusFailed:
            NSLog(@"Export Failed: %@",[exporter error]);
            break;
        case AVAssetExportSessionStatusCancelled:
            NSLog(@"Export canceled");
            break;
        case AVAssetExportSessionStatusCompleted:
            NSLog(@"Export successfully");
            break;
        default:
            break;
    }
    if (exporter.status != AVAssetExportSessionStatusCompleted){
        NSLog(@"Retry export");
        [self renderMovie];
    }
}];

我的代码有问题还是iOS 5有一些错误？