爲什麼我在setValue中獲取NSUnknownKeyException：forUndefinedKey：？

我正在製作一個小型的iOS應用程序，用於跟蹤我工作時鐘的時間。我對Swift和Xcode很陌生，所以這對我來說是一個挑戰。詳情：我正在使用Xcode 7，目標開發是8.3。我相信我缺少像類文件這樣的小東西。該應用程序在啓動時崩潰。爲什麼我在setValue中獲取NSUnknownKeyException：forUndefinedKey：？

錯誤：

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

代碼：

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!" 
    } 
}