使用@IBInspectable设置Bool失败

Question

这是我的班级：

class ViewController: UIViewController {

    @IBInspectable var double: Double!
    @IBInspectable var string: String!


    override func viewDidLoad() {
        super.viewDidLoad()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

}

我在故事板中设置double和string

我运行应用程序。可以正确设置string，但double失败。控制台输出：

Failed to set (double) user defined inspected property on (Test1.ViewController): [<Test1.ViewController 0x7f892ad25910> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key double.

可以告诉我为什么设置失败了吗？谢谢！