Swift mark or uncheck checkbox

I have two buttons modified to work as a checkbox by a subclass ( @IBOutlet siCheckbox and noCheckbox ) if I set a checkbox the other is automatically unmarked the problem is that to say if I press "yes" once uncheck the "no", but if I press the "yes" again it marks out itself and marks the "no"

this is the code

import UIKit

class Paso1: UIViewController, CheckBoxDelegate   {


    @IBOutlet weak var siCheckBox: CheckBox!

    @IBOutlet weak var noCheckBox: CheckBox!



    override func viewDidLoad() {
        super.viewDidLoad()


        self.siCheckBox.delegate = self
        self.noCheckBox.delegate = self}

    func checkBoxDidChange(checkbox: CheckBox) {
    if checkbox == self.siCheckBox {
        self.noCheckBox.isChecked = !checkbox.isChecked

    } else {
        self.siCheckBox.isChecked = !checkbox.isChecked

    }}

And this is the subclass that I have assigned to both buttons

    protocol CheckBoxDelegate {
    func checkBoxDidChange(checkbox: CheckBox) -> Void }

   import UIKit

   class CheckBox: UIButton {

    // Images
    let checkedImage = UIImage(named: "check-greenb")! as UIImage
    let uncheckedImage = UIImage(named: "check-baseb")! as UIImage

    var delegate: CheckBoxDelegate?
    // Bool property
    var isChecked: Bool = false {
        didSet{
            if isChecked == true {
                self.setImage(checkedImage, forState: .Normal)
            } else {
                self.setImage(uncheckedImage, forState: .Normal)
            }
        }
    }

    override func awakeFromNib() {
        self.addTarget(self, action: #selector(CheckBox.buttonClicked(_:)), forControlEvents: UIControlEvents.TouchUpInside)
        self.isChecked = false
    }

    func buttonClicked(sender: UIButton) {
        isChecked = !isChecked
        self.delegate?.checkBoxDidChange(self)
    }}