[WPF 學習] 6 求助：Ellipse的DataTrigger 失效

<StackPanel Width="200">
        <TextBox x:Name="TBS" Text="a" />
        <TextBlock >
            <TextBlock.Style>
                <Style TargetType="TextBlock">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding ElementName=TBS,Path=Text}" Value="a">
                            <Setter Property="Text" Value="Data Trigger is raised"></Setter>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </TextBlock.Style>
        </TextBlock>
        <Ellipse Width="100" Height="100" StrokeThickness="5" Stroke="Red">
            <Ellipse.Style>
                <Style TargetType="Ellipse">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding ElementName=TBS,Path=Text}" Value="a">
                            <Setter Property="Stroke" Value="Black"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Ellipse.Style>
        </Ellipse>
    </StackPanel>

結果：

TextBlock的DataTrigger觸發了有效果，Ellipse的DataTrigger也應該觸發了，可是無效，有誰知道緣由？