Checking country code at sales order entry - BPM query with table jo

I realize I'm asking questions only a small minority are going to
know the potential answer to, but I'm trying to create a BPM that
disallows an order to be entered if someone has an non-US country and
selects UPS Ground.

However, since the country code is embedded in the Customer table and
not readily accessible in the Sales Order, I'm trying to create a
query in the BPM to join the current CustNum across and then compare
the CountryNum.

Something like:

for each OrderHed where OrderHed.OrderNum = '%orderNum%' AND
OrderHed.ShipViaCode = 'UPS' AND OrderHed.CustNum = Customer.CustNum
AND Customer.CountryNum <> '1'

But I get the following error:

Customer CustNum must be a quoted constant or an unabbreviated,
unambiguous buffer/field reference for buffers known to query . (7328)

Anyone tried to do something similar?