Why don't I get any text in the group header

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WPF

Hi
When I run the following code, I don't get any text in the group header.

The data behind is a datatable, and all the records show up in the listbox according to the specification in the datatemplate (itemTemplate), but when I try to create the header in the same way as I do the binding in the datatemplate for the itemTemplate I only get the color styling right.

Why don't I get any text in my group header?

Code:

<Window.Resources>
    <src:KontaktListe x:Key="cList" />
</Window.Resources>
<Grid>
    <Grid.Resources>
        <CollectionViewSource Source="{StaticResource cList}" x:Key="cvs">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="City"/>
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>
        <DataTemplate x:Key="categoryTemplate">
            <TextBlock Text="{Binding Path=City}" FontWeight="Bold"
                       Background="Beige" Margin="0,0,0,0"
                       Foreground="Black"/>
        </DataTemplate>
        <DataTemplate x:Key="itemTemplate">
            <Grid Margin="4">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="50" />
                    <ColumnDefinition Width="100" />
                    <ColumnDefinition Width="50" />
                    <ColumnDefinition Width="*" />
                </Grid.ColumnDefinitions>
                <TextBlock Text="{Binding Path=FirstName}"  Grid.Column="0" />
                <TextBlock Text="{Binding Path=LastName}"  Grid.Column="1" />
                <TextBlock Text="{Binding Path=ZipCode}" FontWeight="Bold" Grid.Column="2" />
                <TextBlock Text="{Binding Path=City}" Grid.Column="3" />
            </Grid>
        </DataTemplate>
    </Grid.Resources>
    <ListBox Name="MyContacts"
             ItemTemplate="{StaticResource itemTemplate}"
             ItemsSource="{Binding Source={StaticResource cvs}}">
        <ListBox.GroupStyle>
            <GroupStyle HeaderTemplate="{StaticResource categoryTemplate}" />
        </ListBox.GroupStyle>
    </ListBox>
</Grid>

/Stig