Filtering from more than one table

Hi,

I am trying to make a list of Sales Lines, filtered on Shipment Date (sales line table), Customer No (sales line table), and also the Country the item came from (Purchase Header table). I think I can link the Document No field from the Sales Line to the Customer Order No field on the Purchase Header, but I keep getting VALUE errors whenever I try to add the filter based on Purchase Header table.

Here is the formula I am using:
=NL("rows","sales line",,"sell-to customer no.","C123456","shipment date","01/02/13..31/01/14",NL("Filter","Purchase Header","Customer Order No.","Buy-from county","China"))

I have also tried using "Link=" but I couldnt get that to work either :(

If anyone could tell me which bit I'm doing wrong it would be much appreciated!

Thank you