Error message when using outer join for a form data source

I added the purchLine table as a form data source to the item arrival journal.

JoinSource: WMSJournalType.

LinkType: OuterJoin

When I open the item arrival journal I get the error message:

"Cannot select a record in Purchase lines (PurchLine). The join does not contain any link between the joined tables in the WHERE clause."

I don't get an error message when I use an InnerJoin though. There is a relation between the two tables. Why would it work for an InnerJoin but not for an OuterJoin? How can I solve this? Thanks for any hints you can give me!