无法在UIViewControllers之间强制转换

Question

我正在尝试使用分段控件在两个视图控制器之间切换。这是profileContainerViewController的代码，它控制开关：

class profileContainerViewController: UIViewController {



@IBOutlet weak var segmentedControl: UISegmentedControl!

let viewControllerIdentifiers = ["first", "second"]


@IBAction func valueChanged(sender: AnyObject) {

    var newController = storyboard?.instantiateViewControllerWithIdentifier(viewControllerIdentifiers[sender.selectedSegmentIndex]) as! UIViewController
    let oldController = childViewControllers.last as! UIViewController

    oldController.willMoveToParentViewController(nil)
    addChildViewController(newController)
    newController.view.frame = oldController.view.frame

    transitionFromViewController(oldController, toViewController: newController, duration: 0.25, options: .TransitionCrossDissolve, animations:{ () -> Void in
        // nothing needed here
        }, completion: { (finished) -> Void in
            oldController.removeFromParentViewController()
            newController.didMoveToParentViewController(self)
    })

}
}

我还有一个嵌入了第一个View Controller的UIContainerView：

但是，当我加载视图时，我收到此错误：

"Could not cast value of type 'profileContainerViewController' (0x107917830) to 'userProfileViewController' (0x107917620)."

（userProfileViewController是第一个必须加载的ViewController。）