Sent Messages has an group by error with Postgres

When clicking on the Sent Messages link in the Private Messages block I get the following errors ...

* warning: pg_query() [function.pg-query]: Query failed: ERROR: column "pm.subject" must appear in the GROUP BY clause or be used in an aggregate function in /var/www/testd.msc.org.nz/includes/database.pgsql.inc on line 139.
* user warning: query: SELECT pmi.thread_id, pm.subject, MAX(pm.timestamp) as timestamp FROM pm_message pm INNER JOIN pm_index pmi ON pm.mid = pmi.mid WHERE (pmi.uid = 1) AND (pm.author = 1) AND (pmi.deleted = 0) GROUP BY pmi.thread_id ORDER BY timestamp desc LIMIT 25 OFFSET 0 in /var/www/testd.msc.org.nz/sites/default/modules/privatemsg/privatemsg.module on line 274.

Still learning about PHP and drupal development, so I'm having trouble working out where the error is :-(