"This class is not key value coding-compliant for the key" error in Swift

I have been getting this error message in Swift:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<Faceit.ViewController 0x7f8f72501e40> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key faceview.'

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]

}