Bind to ControlTemplate onwer's property

I have a ControlTemplate, which I use as Validation.ErrorTemplate:

            <ControlTemplate x:Key="ErrorTemplate">
                <Border BorderBrush="Red" BorderThickness="2">
                    <Grid>
                        <AdornedElementPlaceholder/>
                        <TextBlock Text="{Binding [0].ErrorContent}" HorizontalAlignment="Right" Margin="100 0 5 0" Foreground="Red" Width="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ActualWidth}" TextWrapping="Wrap"/>
                    </Grid>
                </Border>
            </ControlTemplate>

            <Style TargetType="TextBox" BasedOn="{StaticResource DefaultTextBoxStyle}">
                <Setter Property="TextAlignment" Value="Left"/>
                <Setter Property="MinHeight" Value="50"/>
                <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ErrorTemplate}"/>
            </Style>

I am trying to bind ControlsTemplate's TextBlock's Width to ActualWidth of TextBlock using it by TemplatedParent, but it is not working properly.

Any ideas why?

You could bind to the ActualWidth of the AdornedElementPlaceholder:

<Grid>
    <AdornedElementPlaceholder x:Name="elem"/>
    <TextBlock Text="{Binding [0].ErrorContent}" HorizontalAlignment="Right"
               Foreground="Red" Width="{Binding Path=ActualWidth, ElementName=elem}" TextWrapping="Wrap"/>
</Grid>