binding of datagridcomboboxcolumn problem

Hi,
i'm a newbie and i am trying to fill a datagrid with some data from my list of classes.

I have 2 classes, clients and kal
in the calvul class i have a klid that contains the id that corresponds with the Id from Clients

the datagrid's datacontext is kal (it show some info + clientsname)
so in my datagrid i want a dropdown with all the clients
but it should show at startup the value thats is in my klid from kal
but i can't figure out how to get this working...

in my code i have
KalenderGrid.DataContext = Kal;
gezinColumn.ItemsSource = Klanten;

my xaml-part looks like this :

<DataGridComboBoxColumn Header="Gezin" Width="100" x:Name="gezinColumn"
DisplayMemberPath="KlNaam" SelectedValuePath="Id" Selector.IsSelected="True"
SelectedValueBinding="{Binding Path=Klanten}" >
<DataGridComboBoxColumn.ElementStyle>
<Style TargetType="{x:Type ComboBox}">
<Setter Property="ItemsSource" Value="{Binding Path=Kal.Period1.klid, RelativeSource={RelativeSource AncestorType={x:Type Window}}}" />
</Style>
</DataGridComboBoxColumn.ElementStyle>
</DataGridComboBoxColumn>

it shows the column, but it is empty, when i click it open, i can see all my clients..

could anyone point my in the right direction ?
thanks in advance