LockablePivot throws InvalidCastException

我正在尝试实现Windows Phone Toolkit-2011年8月（7.1 SDK）版本中的lockablePivot控件。

每次在运行时更改isLocked属性时，向控件添加项模板时，它都会引发InvalidCastException。有什么解决办法吗？

需要帮助。感谢

---

<Grid x:Name="LayoutRoot" Background="Transparent">
    <toolkit:LockablePivot Name="Pages" SelectionChanged="Pivot_Changed" ItemsSource="{Binding}">           
        <toolkit:LockablePivot.ItemTemplate>
                <DataTemplate>
                <Grid VerticalAlignment="Stretch">
                    <Button Content="Lock" Click="Button_Click"/>
                    <Image Stretch="Uniform" local:WP7ImageZoomer.IsZoomingEnabled="True" ManipulationStarted="Image_ManipulationStarted" ManipulationCompleted="Image_ManipulationCompleted" ManipulationDelta="Image_ManipulationDelta" CacheMode="BitmapCache">
                        <Image.Source>
                            <BitmapImage x:Name="Bit" UriSource="{Binding Img}" DownloadProgress="BitmapImage_DownloadProgress" CreateOptions="BackgroundCreation"/>
                        </Image.Source>
                    </Image>
                </Grid>
            </DataTemplate>
        </toolkit:LockablePivot.ItemTemplate>
    </toolkit:LockablePivot>
</Grid>

C#：

private void Image_ManipulationStarted(object sender, System.Windows.Input.ManipulationStartedEventArgs e)
    {
        this.Pages.IsLocked = true;// Throws InvalidCastException here
    }
    private void Image_ManipulationCompleted(object sender, System.Windows.Input.ManipulationCompletedEventArgs e)
    {
        this.Pages.IsLocked = false;
    }
    private void Image_ManipulationDelta(object sender, System.Windows.Input.ManipulationDeltaEventArgs e)
    {
        //this.Pages.IsLocked = true;
    }