AVPlayer not playing local mp4

I am trying to use an AVPlayer to play a video that has been recorded in my app. However, the player won't play the video. I know for a fact that this is a properly recorded mp4 file, because I can take it and play it on my Mac just fine. Here's the setup for the player:

let documents = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first!
let URL = NSURL(fileURLWithPath: "tempVideo", relativeToURL: documentsDirectory)
let asset = AVAsset(URL: URL)
let item = AVPlayerItem(asset: asset)

//videoPlayer is a property on the view controller being used
videoPlayer = AVPlayer()

//videoPlayerLayer is a property on the view controller being used
videoPlayerLayer = AVPlayerLayer()
videoPlayerLayer.frame.size = view.frame.size
videoPlayerLayer.backgroundColor = UIColor.redColor().CGColor

view.layer.addSublayer(videoPlayerLayer!)

//wait 5 seconds

videoPlayer.play()

I know for sure that the videoPlayer is, in fact, ready to play, because I've checked its status property. I also know that videoPlayerLayer has properly been added to view.layer because its visible and takes up the whole screen. When I call videoPlayer.play(), the music playing on the device stops, but videoPlayerLayer doesn't show anything.

Any ideas? Thank you in advance for the help!