Thread Break issue

mutating func randomFact() -> String {
    var arrayCount = UInt32(factsArray.count)

    var randomNumber = Int(arc4random_uniform(arrayCount))
    while contains(countCheck, randomNumber)
    {
        randomNumber = Int(arc4random_uniform(arrayCount))
    }

    countCheck.append(randomNumber)
    return factsArray[randomNumber]
}

func gameOver() -> Bool {
    return ((countCheck.count - 1) == factsArray.count)
}

@IBOutlet weak var funFactLabel: UILabel!

var factBook = FactBook()

override func viewDidLoad() {
    super.viewDidLoad()
    funFactLabel.text = factBook.randomFact()
    // 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.
}

@IBAction func showFunFact() {
        funFactLabel.text = factBook.randomFact()
}