IDataErroInfo not displaying as tool tip

hi,

My problem is error is not displaying as tool tip but display in ContentPresenter! Both codes are given below.

//this is working fine.
<ContentPresenter 
      Grid.Row="1" Grid.Column="2"
      Content="{Binding ElementName=txtCd, Path=(Validation.Errors).CurrentItem.ErrorContent}"
      />     

//but this code is not working 

<Style x:Key="EntryText" TargetType="{x:Type TextBox}">            <Style.Triggers>                <Trigger Property="Validation.HasError" Value="true">                    <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors).CurrentItem.ErrorContent}" />        </Trigger>        </Style.Triggers>        </Style>
<TextBox x:Name="txtCd" Grid.Column="1" Grid.Row="1" Height="25" Width="150" HorizontalAlignment="Left" Text="{Binding CurrentPost.Cd,ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" Style="{StaticResource EntryText}"/>

Looking for a solution, thanks in advance.