Ios Swift POPOPOR错误阴影，如果宽度<；30

这是主控制器，它是故事板上的一个简单的popover序列。问题是如果我将宽度设置为低于50，就会出现奇怪的阴影

class MainViewController: UIViewController, UIPopoverPresentationControllerDelegate
{
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.

}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?)
{
if let popupView = segue.destinationViewController as? UIViewController
{
  if let popup = popupView.popoverPresentationController
  {
    popup.delegate = self
           popupView.preferredContentSize = CGSize(width: 30, height: 250)
          }
}
    }

func adaptivePresentationStyleForPresentationController(controller:      UIPresentationController) -> UIModalPresentationStyle
{
return UIModalPresentationStyle.None
}

}