Add a subtitle with AVMutableComposition

I am trying to use AVMutableComposition on iOS 9 to combine an MP4 video with additional subtitles in WebVTT, but it does not work.

import Foundation
import AVKit
import AVFoundation
func delay(delay:Double, closure:()->()) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue(), closure)
}
class PlayerViewController : UIViewController {
    override func viewDidAppear(animated: Bool) {
       
        let videoAsset = AVURLAsset(URL: NSURL(fileURLWithPath: "/Users/mxey/Documents/Putview/SubtitleSandbox/video.mp4"), options: [:])
        let subtitleAsset = AVURLAsset(URL: NSURL(fileURLWithPath: "/Users/mxey/Documents/Putview/SubtitleSandbox/subtitles.webvtt"), options: [:])
       
        let composition = AVMutableComposition()
       
        try! composition.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoAsset.duration), ofAsset: videoAsset, atTime: kCMTimeZero)
        let subtitleTrack = composition.addMutableTrackWithMediaType(AVMediaTypeSubtitle, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
        try! subtitleTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, subtitleAsset.duration), ofTrack: subtitleAsset.tracks[0], atTime: kCMTimeZero)
        let playerVC = AVPlayerViewController()
        presentViewController(playerVC, animated: false, completion: nil)
        playerVC.player = AVPlayer(playerItem: AVPlayerItem(asset: composition))
        playerVC.player!.play()
       
        delay(5) {
            let item = playerVC.player!.currentItem!
            print(item.error)
        }
    }
}

The AVPlayer shows the strikethrough play button to indicate an error. The output of the print statement is:

Optional(Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSUnderlyingError=0x7f8b2bc67500 {Error Domain=NSOSStatusErrorDomain Code=-12844 "(null)"}, NSLocalizedFailureReason=An unknown error occurred (-12844), NSLocalizedDescription=The operation could not be completed})

If I use AVMediaTypeText instead of AVMediaTypeSubtitle, the video plays, but the subtitles are always visible and cannot be controlled with the usual subtitle selection menu of AVPlayer.