在Silverlight中绑定ComboBox.SelectedItem(更多)

我有一个像这样绑定的ComboBox：

<ComboBox x:Name="PART_CommentaryList" 
    HorizontalAlignment="Left" 
    Margin="3" 
    Itemssource="{Binding Path=CurrentVideo.Commentaries}" 
    SELEctedItem="{Binding Path=CurrentCommentary,Mode=TwoWay}">

CurrentVideo和CurrentCommentary属性都会定期更改.几次后,我收到此错误：

Category: ManagedRuntimeError       
message: System.Argumentexception: Value does not fall within the expected
   range.
   at Ms.Internal.XcpImports.MethodEx(IntPtr ptr,String name,CValue[] cvData)
   at Ms.Internal.XcpImports.MethodPack(IntPtr objectPtr,String methodName,Object[] rawData)
   at Ms.Internal.XcpImportS.UIElement_TransformToVisual(UIElement element,UIElement visual)
   at System.WindowS.UIElement.TransformToVisual(UIElement visual)
   at System.Windows.Controls.Primitives.SELEctor.IsOnCurrentPage(
       Int32 index,Rect& itemsHostRect,Rect& listBoxItemRect)
   at System.Windows.Controls.Primitives.SELEctor.ScrollIntoView(
       Int32 indeX)
   at System.Windows.Controls.Primitives.SELEctor.SetFocusedItem(
       Int32 index,Boolean scrollIntoView)
   at System.Windows.Controls.ComboBox.PrepareContainerForItemOverride(
       DependencyObject element,Object item)
   at System.Windows.Controls.ItemsControl.updateContainerForItem(
       Int32 indeX)
   at System.Windows.Controls.ItemsControl.RecreateVisualChildren()
   at System.Windows.Controls.ItemsControl.RecreateVisualChildren(
       IntPtr unmanagedObj)

这对我来说似乎是一个ComboBox错误.我可以在CurrentCommentary之前验证CurrentVideo是否更改,因此所选项应该始终是列表中的项.

相关,我真的不想要Mode = TwoWay,因为当Itemssource被更改时,SELEctedItem暂时为null,它在我的模型中被设置回来,我实际上并不想要.但绑定根本不起作用(这似乎是另一个错误).