Unbalanced calls to begin/end appearance transitions with UIPercentDrivenInteractiveTransition

I am trying to use UIPercentDrivenInteractiveTransition to allow the user to swipe a new view in from the righthand side.

All works okay apart from the warning; "Unbalanced calls to begin/end appearance transitions ...."

The fromView is connected to the toView in the Storyboard using a Custom Segue called "toRightView"

Here is the handler:

func handleRightOnStagePan(pan: UIPanGestureRecognizer) 
{ 
     //how much distance has been panned
     let translation = pan.translationInView(pan.view!) 

     //translate to percentage based value
     var d = -translation.x / CGRectGetWidth(pan.view!.bounds) 
     d = min(1.0, max(0.0,d)) 
     
     //now deal with different states of geture recogniser
     switch (pan.state) { 
          case UIGestureRecognizerState.Began: 
               //trigger start of transition 
               self.presentingViewController.performSegueWithIdentifier("toRightView", sender: self) 
               break 
          case UIGestureRecognizerState.Changed: 
               //Update progress of transition
               self.updateInteractiveTransition(d) 
               break 
          default:   //.Ended, .Cancelled, .Failed
               if (d > 0.3) { 
                  self.finishInteractiveTransition() 
               } else { 
                  self.cancelInteractiveTransition() 
               } 
      } 
}                                                  

I have seen that other solutions to this warning message suggest that one transition has started before another has finished but in this case a break-point on

performSegueWithIdenfier(...)

Any help with fixing this would be greatly appreciated.