为什么我在setValue中获取NSUnknownKeyException：forUndefinedKey：？

我正在制作一个小型的iOS应用程序，用于跟踪我工作时钟的时间。我对Swift和Xcode很陌生，所以这对我来说是一个挑战。详情：我正在使用Xcode 7，目标开发是8.3。我相信我缺少像类文件这样的小东西。该应用程序在启动时崩溃。为什么我在setValue中获取NSUnknownKeyException：forUndefinedKey：？

错误：

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key ButtonIn.'

代码：

import UIKit 

class ViewController: UIViewController { 

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



    } 
    @IBOutlet weak var Lb1: UILabel! 

    @IBAction func ButtonIn(sender: AnyObject) { 
     self.Lb1.text = "Clocked In!" 
    } 

    @IBAction func ButtonOut(sender: AnyObject) { 
     self.Lb1.text = "Clocked Out!" 
    } 
}