WPF文本框:设置样式时出错

我已经为文本框定义了样式,当文本框为空时,该样式将显示关于输入内容的提示。

这将是我的窗口:

<Grid>
    <Grid.Resources>
        <Style TargetType="{x:Type TextBox}" x:Key="stlHintbox">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Text, RelativeSource={RelativeSource Mode=Self}}" Value="">
                    <Setter Property="Background">
                        <Setter.Value>
                            <VisualBrush Stretch="None">
                                <VisualBrush.Visual>
                                    <TextBlock Text="First name" FontStyle="Italic" Foreground="LightGray" />
                                </VisualBrush.Visual>
                            </VisualBrush>
                        </Setter.Value>
                    </Setter>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Grid.Resources>
    <TextBox Style="stlHintbox" />
</Grid>

抛出的错误是:

'Set property 'System.Windows.FrameworkElement.Style' threw an exception.' Line number '22' and line position '11'.

内部除外:

"'stlHintbox' is not a valid value for property 'Style'."

样式放置在内部时正常工作 <TextBox.Style> ,那我做错什么了?