SQL & PL/SQL

Hi All,
Hopefully someone out there can help me out with this one. I have a sql query that performs a sub-query as part of the select cause combined with an aggregate function and is giving me a "not a group by expression".

I've created a simplified example

 SELECT COUNT(*)                     ,
  ai.invoice_num invoice_number      ,
  ai.description invoice_description ,
  (SELECT poh.po_header_id
       FROM po_headers poh ,
      po_distributions pod
      WHERE pod.po_distribution_id = aid.po_distribution_id
    AND pod.po_header_id           = poh.po_header_id
    ) po_number
     FROM ap_invoice_distributions aid ,
    ap_invoices ai
    WHERE ai.invoice_id = 1476932
  AND aid.project_id    = 8608   
  AND aid.task_id       = 462202  
  AND ai.invoice_id     = aid.invoice_id
 GROUP BY ai.invoice_num,
                ai.description;

I'm pretty sure that it is expecting me to add the po_number to the group by clause but this doesn't work and gives the error "invalid identifier".

Can anyone suggest a way to re-gig the query so that I can add po_number to my group by clause.

Kris