title: fatal error: unexpectedly found nil while unwrapping an Optional value
body: <p>I am trying to run this code but I keep on getting this error:</p>
  
  <blockquote>
    <p>fatal error: unexpectedly found nil while unwrapping an Optional value</p>
  </blockquote>
  
  <p>I don't understand what it means or why I'm getting it. Any hint?</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  
  var lastNumber: String = ""
  @IBOutlet var answerField: UILabel
  @IBOutlet var operaterLabel: UILabel
  
  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.
  }
  
  @IBAction func buttonTapped(theButton: UIButton) {
      if answerField.text == "0"
      {
          answerField.text = theButton.titleLabel.text
      }
      else
      {
          answerField.text = answerField.text + theButton.titleLabel.text
      }
  }
  
  @IBAction func plusTapped(theButton: UIButton) {
      // error is talking about the next line
      if operaterLabel.text == ""
      {
          operaterLabel.text = "+"
          lastNumber = answerField.text
          answerField.text = "0"
      }
      else
      {
          enterTapped(nil)
          operaterLabel.text = "+"
      }
  
  }
  
  @IBAction func minusTapped(theButton: UIButton) {
      if operaterLabel.text == ""
      {
          operaterLabel.text = "-"
          lastNumber = answerField.text
          answerField.text = "0"
      }
      else
      {
          enterTapped(nil)
          operaterLabel.text = "-"
      }
  
  }
  
  @IBAction func clearTapped(AnyObject) {
      answerField.text = "0"
      operaterLabel.text = ""
      lastNumber = ""
  
  }
  
  @IBAction func enterTapped(AnyObject?) {
  
      var num1 = lastNumber.toInt()
      var num2 = answerField.text.toInt()
      if !num1 || !num2
      {
          showError()
          return
      }
      var answer = 0
      if operaterLabel.text == "-"
      {
          var answer = num1! - num2!
      }
      else if operaterLabel.text == "+"
      {
          var answer = num1! + num2!
      }
      else
      {
          showError()
          return
      }
      answerField.text = "\(answer)"
  
  }
  
  func showError()
  {
      println("Ther was an error")
  }
  }
  </code></pre>