Error 1055 from IndexPager::buildQueryInfo (UnreviewedPagesPager), 'wikidb.unittest_page.page_namespace' isn't in GROUP BY

SELECT  page_namespace,page_title,page_len,page_id,MIN(rev_timestamp) AS creation  FROM `unittest_page` FORCE INDEX (page_name_title) LEFT JOIN `unittest_flaggedpages` ON ((fp_page_id=page_id)) LEFT JOIN `unittest_revision` FORCE INDEX (rev_page_timestamp) ON ((rev_page=page_id))   WHERE (fp_page_id IS NULL) AND page_namespace = 0 AND page_is_redirect = 0  GROUP BY page_title ORDER BY page_title LIMIT 51