ios - “numberOfLines”时出错

Question

class ViewController: UIViewController {
    @IBOutlet weak var btn: UIButton!
    override func viewDidLoad() {
        // Error on following line: "Thread 1: EXC_BAD_INSTRUCTION"
        btn.titleLabel?.numberOfLines = 1;
        btn.titleLabel?.adjustsFontSizeToFitWidth = true
        btn.titleLabel?.lineBreakMode = .byClipping
        super.viewDidLoad()
    }
}

我该怎么办？发生此错误时，即使我的模拟器也无法打开。