WPF Binding to Tooltip

Not sure whats doing here, but the binding works for the label in the data template but not the tool tip. Any help will be appreciated.

                    <DataTemplate DataType="Label">
                    <StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
                        <StackPanel.ToolTip>
                            <ToolTip DataContext="{Binding Path=PlacementTarget,
       RelativeSource={x:Static RelativeSource.Self}}">
                                <TextBlock Text="{Binding Path=DataContext.Description}" />
                            </ToolTip>
                        </StackPanel.ToolTip>
                        <Image Source="{StaticResource ApplicationInfoS}" 
                               Margin="0 0 5 0" Stretch="None"
                               HorizontalAlignment="Left" />
                        <Label Style="{StaticResource lblTextContent}" 
                               Padding="5 0 0 0"
                               Content="{Binding Path=DataContext.Description, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListViewItem}}}"/>
                    </StackPanel>
                </DataTemplate>

BTW the DataTemplate is used in Listview. "Description" property exists on the view model bound to the list view.

The message I get in the output window in VS2010 is:

System.Windows.Data Error: 39 : BindingExpression path error: 'Description' property not found on 'object' ''String' (HashCode=-466763399)'. BindingExpression:Path=DataContext.Description; DataItem='StackPanel' (Name=''); target element is 'TextBlock' (Name=''); target property is 'Text' (type 'String')

UPDATE

I have given up for now. Using the following hack for the time being:

Add a Tag to the StackPanel and Bind "Description" to it

    <StackPanel Orientation="Horizontal" HorizontalAlignment="Left" Tag="{Binding Path=DataContext.Description, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListViewItem}}}">

Bind the ToolTip to the Tag. Yes a hack but it works.

<StackPanel.ToolTip>
  <ToolTip DataContext="{Binding Path=PlacementTarget, RelativeSource={x:Static RelativeSource.Self}}">
    <TextBlock Text="{Binding Path=Tag}" />
  </ToolTip>
</StackPanel.ToolTip>

Cheers

Mike