Select statement in form personalization

Hello experts

I have a requirement where i need to pull a single row value from a select statement . I developed one personalization, but couldn't able to run my select statment.

Evert time it is giving null, because i may not following proper syntax.

Action tab of form personalization:

Property Name : value

=(select distinct comments

from apps.ap_invoice_lines_all aila, apps.ap_invoices_all aia, apps.po_headers_all pha

where  aia.invoice_id = aila.invoice_id

and      aila.po_header_id =  :LINE_SUM_FOLDER.PO_HEADER_ID

and      pha.po_header_id = aila.po_header_id

)

Can you suggest me the correct syntax , so that my query starts giving rows.