windows 8 MSDN Examples 中一个奇怪的问题

http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.media.animation.repositionthemetransition.aspx

<Button Content="Remove Rectangle" Click="RemoveButton_Click"/>

<ItemsControl Grid.Row="1" x:Name="rectangleItems">
    <ItemsControl.ItemContainerTransitions>
        <TransitionCollection>
            
            <!-- Without this, there would be no animation when items 
                 are removed. -->
            <RepositionThemeTransition/>
        </TransitionCollection>
    </ItemsControl.ItemContainerTransitions>
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <WrapGrid Height="400"/>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    
    <!-- All these rectangles are just to demonstrate how the items
         in the grid re-flow into position when one of the child items
         are removed. -->
    <ItemsControl.Items>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
        <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
    </ItemsControl.Items>
</ItemsControl>

private void RemoveButton_Click(object sender, RoutedEventArgs e)
{
    if (rectangleItems.Items.Count > 0)
        rectangleItems.Items.RemoveAt(0);
}

每点击一次按钮，将删除一个红色的方块。但当方块删除的只剩一个时，再点击按钮删除时会报错“Value does not fall within the expected range.”
很奇怪的提示，明明还有一个，怎么就删不掉呢？难道rectangleItems.Items不能为空吗？试验一下，调用rectangleItems.Items.Clear()方法删除rectangleItems.Items中的所有项，反应正常；又试了下，在XAML中<ItemsControl.Items>中的方块直接都删除，任然可以运行。

不知有没有人也遇到了类似的问题，这到底是什么原因呢？