Amending SQL slightly brings me: ORA-01427: single-row subquery returns more than one row

Summary

ORA-01427

Content

I am using Oracle Fusion, the finance/payables reporting side of things

I have the following code:

    SELECT DISTINCT PHA.SEGMENT1 
    FROM 
        AP_INVOICES_ALL AIA, -- https://docs.oracle.com/en/cloud/saas/financials/19a/oedmf/AP_INVOICES_ALL-tbl.html
        PO_HEADERS_ALL PHA -- https://docs.oracle.com/applications/farel12/procurementop_gs/OEDMP/PO_HEADERS_ALL_tbl.htm
    WHERE
        AIA.PO_HEADER_ID = PHA.PO_HEADER_ID 
        AND AIA.PO_HEADER_ID IS NOT NULL 
        AND AIA.INVOICE_NUM = XTE.TRANSACTION_NUMBER 
        AND AIA.INVOICE_ID = XTE.SOURCE_ID_INT_1

This "works" in that it returns no errors. However it's looking at the wrong table so wrong data is being brought back. No biggie, we have identified which table it is.