I wrote a program in Swift.I want to merge a video with an audio file, but got this error.
"failed Error Domain=AVFoundationErrorDomain Code=-11838 "Operation Stopped" UserInfo=0x17da4230 {NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The operation is not supported for this media.}"
code
func mergeAudio(audioURL: NSURL, moviePathUrl: NSURL, savePathUrl: NSURL) {
    var 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 [NSObject : AnyObject])
    let audioAsset = AVURLAsset(URL: audioURL, options: option as [NSObject : AnyObject])
    let tracks = sourceAsset.tracksWithMediaType(AVMediaTypeVideo)
    let audios = audioAsset.tracksWithMediaType(AVMediaTypeAudio)
    if tracks.count > 0 {
        let assetTrack:AVAssetTrack = tracks[0] as! AVAssetTrack
        let assetTrackAudio:AVAssetTrack = audios[0] as! AVAssetTrack
        let audioDuration:CMTime = assetTrackAudio.timeRange.duration
        let audioSeconds:Float64 = CMTimeGetSeconds(assetTrackAudio.timeRange.duration)
        trackVideo.insertTimeRange(CMTimeRangeMake(kCMTimeZero,audioDuration), ofTrack: assetTrack, atTime: kCMTimeZero, error: nil)
        trackAudio.insertTimeRange(CMTimeRangeMake(kCMTimeZero,audioDuration), ofTrack: assetTrackAudio, atTime: kCMTimeZero, error: nil)
    }
    var assetExport: AVAssetExportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetPassthrough)
    assetExport.outputFileType = AVFileTypeMPEG4
    assetExport.outputURL = savePathUrl
    self.tmpMovieURL = savePathUrl
    assetExport.shouldOptimizeForNetworkUse = true
    assetExport.exportAsynchronouslyWithCompletionHandler { () -> Void in
        switch assetExport.status {
        case AVAssetExportSessionStatus.Completed:
            let assetsLib = ALAssetsLibrary()
            assetsLib.writeVideoAtPathToSavedPhotosAlbum(savePathUrl, completionBlock: nil)
            println("success")
        case  AVAssetExportSessionStatus.Failed:
            println("failed \(assetExport.error)")
        case AVAssetExportSessionStatus.Cancelled:
            println("cancelled \(assetExport.error)")
        default:
            println("complete")
        }
    }
}
In my idea media type like mpeg4 is wrong. Where is the problem? What am i missing?