访问按钮目标投掷错误

使用下面的代码，当我按下按钮时出现错误。我不确定为什么。错误是无法识别的选择器发送到实例，并以“终止为NSException类型的未捕获异常”终止。任何帮助肯定表示赞赏。谢谢

class WordSearchWindow: UIViewController {

let boardsize = 10

override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(false)
    buttonGridder()
}

func buttonGridder() {
    for x in 0..<self.boardsize {
        for y in 0..<self.boardsize{
            let sizer = ButtonGrid.frame.width
            let buttonSize:CGFloat = sizer / CGFloat(self.boardsize)
            let letterButton = WordButton(column: x, row: y, buttonSize: buttonSize, buttonMargin: 0)
            letterButton.addTarget(self, action: "letterFirst", forControlEvents: .TouchDown)
            letterButton.addTarget(self, action: "letterSecond", forControlEvents: .TouchUpInside)
            self.ButtonGrid.addSubview(letterButton)
        }
    }
}

func letterFirst(sender: WordButton) {
    print("hello1")
}

func letterSecond(sender: WordButton) {
    print("hello2")
}
}