I've got a grid which shows a column based on a converter, defined like this:

<DataGridTextColumn Header="Invoice #"
                    Width="*" 
                    Binding="{Binding crt_invfk, Converter={StaticResource Inv_PkToNumberConverter}}"                                                     
                    x:Name="txtInv_Number" IsReadOnly="False"/>

This takes the crt_invfk and looks up the actual invoice number and returns that for display. If I click on the column header the sorting gets done on crt_invfk, whereas I need it to sort on the Invoice number.

How do I achieve this?