I have made a custom field "PO number" under Sales Order.

I have made the same custom field under Invoices and set the default value to ${salesorder.cf_po} so that when I make an invoice the PO number from the sales order is automatically filled in.

This works fine when I convert the sales order to an invoice. However it doesn't work when I create an invoice from "Projects" --> "New transaction" --> "Create invoice" although the sales order is linked to the project. What do I wrong?