给控件绑定事件函数的时候，报错

在手动给控件添加事件的时候，报错
代码如下

class ViewController: UIViewController {

    @IBOutlet weak var button: UIButton!
    @IBOutlet weak var lable: UILabel!
    override func viewDidLoad() {
        super.viewDidLoad()
        button.addTarget(self, action: "buttonDown:", forControlEvents: .TouchDown)
        // 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.
    }
    func buttonDown(sender:AnyObject)
    {
        lable.text = "哈哈哈，来个hellow，world"
    }
}

报错如下

fatal error: unexpectedly found nil while unwrapping an Optional value

后来把控件都删除，重新拖一下就好了。。。。