title: "This class is not key value coding-compliant for the key" error in Swift
body: <p>I have been getting this error message in Swift:</p>
  
  <blockquote>
    <p>Terminating app due to uncaught exception 'NSUnknownKeyException', reason: <code>'[&lt;Faceit.ViewController 0x7f8f72501e40&gt; setValue:forUndefinedKey:]:</code> this class is not key value coding-compliant for the key faceview.'</p>
  </blockquote>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
      @IBOutlet weak var faceView: FaceView! {
          didSet{
              updateUI()
          }
      }
  
      var expression = FacialExpression(eyes: .closed, mouth: .frown) {
          didSet {
              updateUI()
          }
      }
  
      private func updateUI() {
          switch expression.eyes {
          case .open:
              faceView?.eyesOpen = true
          case .closed:
              faceView?.eyesOpen = false
          case .squinting:
              faceView?.eyesOpen = false
          }
          faceView?.mouthCurvature = mouthCurvatures[expression.mouth] ?? 0.0
      }
  
      private let mouthCurvatures = [FacialExpression.Mouth.grin:0.5,.frown: -1.0,.smile:1.0,.neutral:0.0,.smirk:-0.5]
  
  }
  </code></pre>