GameScene misses initializers - swift

Question

I get an error that "Class 'GameScene' has no initializers" when I try to add a level selection to my game.
I present the scene in GameViewController.swift like this:
override func viewDidLoad() {
super.viewDidLoad()
if let scene = GameScene.level(1) {
// Configure the view.
let skView = self.view as! SKView
skView.showsFPS = true
skView.showsNodeCount = true
skView.showsPhysics = true
/* Sprite Kit applies additional optimizations to improve rendering performance */
skView.ignoresSiblingOrder = true
/* Set the scale mode to scale to fit the window */
scene.scaleMode = .AspectFill
skView.presentScene(scene)
}
}
And in my GameScene.swift I wanted to handle the level selection like this (got this from Ray Wenderlich iOS Game Development Books)
class func level(levelNum: Int) -> GameScene? {
let scene = GameScene(fileNamed: "Level\(levelNum)")!
scene.currentLevel = levelNum
scene.scaleMode = .AspectFill
return scene
}
But then I get the error as I mentioned and I dont know how to fix this issue :-? Are there any other ways to make a level selection? Currently I have for each level one sks file, which is something I find really handy, since I enables me to lay out the levels visually.
Any recommendations and/or help?
Thanks in advance