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

 1 <Button Content="Remove Rectangle" Click="RemoveButton_Click"/>
 2 
 3 <ItemsControl Grid.Row="1" x:Name="rectangleItems">
 4     <ItemsControl.ItemContainerTransitions>
 5         <TransitionCollection>
 6             
 7             <!-- Without this, there would be no animation when items 
 8                  are removed. -->
 9             <RepositionThemeTransition/>
10         </TransitionCollection>
11     </ItemsControl.ItemContainerTransitions>
12     <ItemsControl.ItemsPanel>
13         <ItemsPanelTemplate>
14             <WrapGrid Height="400"/>
15         </ItemsPanelTemplate>
16     </ItemsControl.ItemsPanel>
17     
18     <!-- All these rectangles are just to demonstrate how the items
19          in the grid re-flow into position when one of the child items
20          are removed. -->
21     <ItemsControl.Items>
22         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
23         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
24         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
25         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
26         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
27         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
28         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
29         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
30         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>
31     </ItemsControl.Items>
32 </ItemsControl>

1 private void RemoveButton_Click(object sender, RoutedEventArgs e)
2 {
3     if (rectangleItems.Items.Count > 0)
4         rectangleItems.Items.RemoveAt(0);
5 }

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？