Workflow to PI based on Expenditure Type

Content

The current invoice workflow configuration automatically bypasses approvals from the PI and Cost Center when the invoice is tied to a purchase order. We have a new business requirement for our PI approval. If the invoice is on a purchase order AND a specific Expenditure Type/Natural Account is used, the PI should approve the invoice. So far, everything I have attempted comes back unable to find a match for the ruleset. The existing rule indicates:

Invoice Header.PO Number is null
Invoice Header.Source isn't "B2B XML INVOICE"
Invoice Distribution.Project ID isn't null

I tried some variations, but I'd think I can group those rules, then add an additional OR section for PO Number isn't null and Invoice Distribution.Expenditure Type Name is "category name" but that isn't working. Any ideas on some variable that should be used. I have also tried with tree mode and advanced rules, but it never finds and assignment. Thanks in advance!