iOS AVFoundation，合并音频和视频时出错11820

Question

我使用此方法合并音频和视频文件，其大小与先前录制和分开存储的大小相同。

func mergeVideoAudio(audioURL: NSURL, moviePathUrl: NSURL, savePathUrl: NSURL) {
    let composition = AVMutableComposition()
    let trackVideo:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())
    let trackAudio:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
    let option = NSDictionary(object: true, forKey: "AVURLAssetPreferPreciseDurationAndTimingKey")
    let sourceAsset = AVURLAsset(URL: moviePathUrl, options: option as? [String : AnyObject])
    let audioAsset = AVURLAsset(URL: audioURL, options: option as? [String : AnyObject])

    let video = sourceAsset.tracksWithMediaType(AVMediaTypeVideo)
    let audio = audioAsset.tracksWithMediaType(AVMediaTypeAudio)

    let assetTrack:AVAssetTrack = video[0] as AVAssetTrack
    let assetTrackAudio:AVAssetTrack = audio[0] as AVAssetTrack

    let videoDuration:CMTime = assetTrack.timeRange.duration
    let audioDuration:CMTime = assetTrackAudio.timeRange.duration

    do {
        try trackVideo.insertTimeRange(CMTimeRangeMake(kCMTimeZero,videoDuration), ofTrack: assetTrack, atTime: kCMTimeZero)
        try trackAudio.insertTimeRange(CMTimeRangeMake(kCMTimeZero, audioDuration), ofTrack: assetTrackAudio, atTime: kCMTimeZero)
        } catch{
            print("error while merging audio and video")
        }
    }

    let assetExport: AVAssetExportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality)!
    assetExport.outputFileType = AVFileTypeQuickTimeMovie
    assetExport.outputURL = savePathUrl
    assetExport.shouldOptimizeForNetworkUse = true
    assetExport.exportAsynchronouslyWithCompletionHandler({
        () -> Void in
        switch assetExport.status {
        case AVAssetExportSessionStatus.Completed:        
            print("completed")                
        case  AVAssetExportSessionStatus.Failed:
            print("failed \(assetExport.error)")
        case AVAssetExportSessionStatus.Cancelled:
            print("cancelled \(assetExport.error)")
        default:
            print("default export status")
        }
    })
}

有时（每8-9次捕获）它会给我这个错误：

failed Optional(Error Domain=AVFoundationErrorDomain Code=-11820 "Cannot Complete Export" UserInfo={NSLocalizedDescription=Cannot Complete Export, NSLocalizedRecoverySuggestion=Try exporting again.})

我尝试过更改视频编码，音频和视频长度，导出选项等，但没有成功。

合并后，我用AVPlayer显示结果预览，当发生此错误时，解码时显然会失败。