WPF，这个动画为什么报错呢？

<Window.Resources>

        <Storyboard x:Key="aa">

            <DoubleAnimation Storyboard.TargetProperty="Ellipse.Width" From="0" To="20" Duration="0:0:2"/>

            <DoubleAnimation Storyboard.TargetProperty="Ellipse.Height" From="0" To="20" Duration="0:0:2"/>

            <DoubleAnimation Storyboard.TargetProperty="Ellipse.StrokeThickness" From="2" To="8" Duration="0:0:2"/>

            <DoubleAnimation Storyboard.TargetProperty="Ellipse.Opacity" From="1" To="0" Duration="0:0:2"/>

        </Storyboard>

    </Window.Resources>

    <Grid HorizontalAlignment="Left" Height="619" VerticalAlignment="Top" Width="1168.25">

        <Ellipse Fill="White" HorizontalAlignment="Left" Height="21.667" Margin="197,39,0,0" VerticalAlignment="Top" Width="21.667" Stroke="#FFF9EFEF">

            <Ellipse.Triggers>

                <EventTrigger RoutedEvent="Ellipse.Loaded">

                    <EventTrigger.Actions>

                        <BeginStoryboard Storyboard="{StaticResource aa}"/>

                    </EventTrigger.Actions>

                </EventTrigger>

            </Ellipse.Triggers>

        </Ellipse>

    </Grid>

为什么报错呢？
------解决方案--------------------
Storyboard.TargetProperty="Ellipse.Width"
改成
Storyboard.TargetProperty="Width"

其他几个，做同样的修改。