BUG #10405: Sum not working with left join

The following bug has been logged on the website:

Bug reference: 10405
Logged by: Zoltan Jozsa
Email address: cousinka(at)gmail(dot)com
PostgreSQL version: 9.1.12
Operating system: Linux
Description:

Hello.
I am a programmer from transylvania, i have a task where i have to get a
list of offer requests ordered by rate, but it's not working for me.

I have written an SQL:
SELECT
OFREQD.offer_request_id,
OFREQD.offer_text,
OFREQD.offer_date,CD.company_name, ( sum( orr.rate ) ) AS offer_rating

FROM offer_request_data AS OFREQD
JOIN offer_message_data AS OFMD ON ( OFREQD.offer_request_id =
OFMD.offer_request_id )
JOIN company_data AS CD ON ( OFMD.recipient_company_id = CD.company_id )
LEFT JOIN offer_request_rating orr ON orr.offer_request_id =
OFREQD.offer_request_id

WHERE ( OFREQD.activity_id = 3239 OR OFMD.activity_id=3239 )
AND OFREQD.is_checked = 1
AND OFREQD.is_spam=0

GROUP BY OFREQD.offer_request_id,OFREQD.offer_text,CD.company_name
ORDER BY offer_rating DESC

OFFSET 0 LIMIT 10

Please take a look on this SQL, and let me know if something is wrong in
SQL, or what can I do to do what the Client ask.

If in the SELECT is SUM, then the server returns NULL as offer_rating for
every row,
if is COUNT then works fine.

Thank you,
Cousin!