"Cannot assign to property: "playlist" is a let constant"

@IBOutlet weak var playListImage0: UIImageView!
@IBOutlet weak var playListImage1: UIImageView!
@IBOutlet weak var playListImage2: UIImageView!
@IBOutlet weak var playListImage3: UIImageView!
@IBOutlet weak var playListImage4: UIImageView!
@IBOutlet weak var playListImage5: UIImageView!


var playListArray: [UIImageView] = []

override func viewDidLoad() {
    super.viewDidLoad()

    playListArray += [playListImage0, playListImage1, playListImage2, playListImage3, playListImage4, playListImage5]

    for index in 0..<playListArray.count {
        let playlist = PlayList(index: index)
        let playListImage = playListArray[index]

        playListImage.image = playlist.icon
        playListImage.backgroundColor = playlist.backgroundColor

    }

    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "showPlayListDetail" {

        let playListImage = sender!.view as! UIImageView
        if let index = playListArray.indexOf(playListImage)
        {
            let detailListController = segue.destinationViewController as! DetailViewController
            detailListController.playlist = PlayList(index: index)


        }

    }
}


@IBAction func imageTap(sender: AnyObject) {
    performSegueWithIdentifier("showPlayListDetail", sender: sender)
}