Erro Occured in data form

 <toolkit:DataField FontWeight="SemiBold"  HorizontalAlignment="Left"      VerticalContentAlignment="Top">
        <toolkit:DataField.Label>
                <StackPanel Orientation="Horizontal" x:Name="stkpnl_labelStar">
                       <TextBlock x:Name="txtblk_labelstar" Text="*"  Foreground="Red"/>
                      <TextBlock x:Name="txtblk_label" Text="Label Name :" />
                </StackPanel>
        </toolkit:DataField.Label>
       <TextBox Text="{Binding label_name, Mode=TwoWay, NotifyOnValidationError=True, 
                ValidatesOnExceptions=True, UpdateSourceTrigger=Explicit}"  
                x:Name="txt_label_name" Width="200" HorizontalAlignment="Left" FontWeight="Normal"/>
</toolkit:DataField>