style trigger for zero length textbox

gistfile1.txt

Would like to have a xaml (view) only method to change the textbox text to "Search ..." when the textbox is empty.

My xaml file has this:

<TextBox Name="searchBox" Text="{Binding Path=TextFilter, UpdateSourceTrigger=PropertyChanged}"/>
  <TextBox.Style>
    <Style TargetType="{x:Type TextBox}">
      <Style.Triggers>
        <DataTrigger Binding="Text.Length, {Binding RelativeSource={RelativeSource = Self}}" Value="0">
          <Setter Property="Text" Value="Search ..." />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </TextBox.Style>
</TextBox>

TextFilter is a string property that may be empty or have a value.

On execution, this error is thrown:

{"'Set property 'System.Windows.DataTrigger.Binding' threw an exception.' Line number '24' and line position '34'."}
Unable to cast object of type 'System.String' to type 'System.Windows.Data.BindingBase'.