UIPushBehavior in swift

Question

I want to implement a UIPushBehavior in Swift, but I 'm always getting following error: ... View item UIImageView should be a descendant of reference view in UIDynamicAnimator ...

I 'm adding the view to the behavior and the behavior to the animator...

@IBAction func push(sender: UITapGestureRecognizer) {

    let pushBehavior = UIPushBehavior(items: [aView], mode: .Instantaneous)
    pushBehavior.magnitude = 0.5
    pushBehavior.action = { [unowned pushBehavior] in
        pushBehavior.dynamicAnimator!.removeBehavior(pushBehavior)
    }
    animator.addBehavior(pushBehavior)

}

I also set the referenceView of the animator to the gameView where the action should happen...

private lazy var animator : UIDynamicAnimator = {

   let lazilyCreatedDynamicAnimator = UIDynamicAnimator(referenceView:self.gameView)
   return lazilyCreatedDynamicAnimator

}()

I hope you can help me figuring out what to do to solve my little problem...