Cannot implicitly convert type Frame to Windows.UI.Xaml.UIElement

Cannot implicitly convert type Frame to Windows.UI.Xaml.UIElement Windows Store project. Unity 5.2.2 p2. In function void InitializeUnity(string args, Windows.ApplicationModel.Activation.SplashScreen splashScreen). Build target : Windows Store -> SDK-> Phone 8.1

         appCallbacks.SetAppArguments(args);
         Frame rootFrame = Window.Current.Content as Frame;

         // Do not repeat app initialization when the Window already has content,
         // just ensure that the window is active
         if (rootFrame == null && !appCallbacks.IsInitialized())
         {
             rootFrame = new Frame();
             Window.Current.Content = rootFrame;
             Window.Current.Activate();

             rootFrame.Navigate(typeof(MainPage));
         }

         Window.Current.Activate();
     }