MySQL Error: Unknown column 'ag_cats.category' in 'order clause'

SELECT gds.*, ctgrs.caption catid_caption, ctgrs.catid, ctgrs.hide c_hide, ctgrs.alias calias, IF(gds.alias!='',gds.alias,gid) gname, (SELECT fid
FROM ag_photos
WHERE
ag_photos.gid= gds.gid
AND
fmain=1) AS fid FROM ag_goods AS gds LEFT JOIN ag_cats ctgrs ON
gds.catid= ctgrs.catid

WHERE
gds.hide=0
AND
gds.inshop=1
AND
ctgrs.hide=0
AND
ctgrs.alias = 'numismatics'
ORDER BY ag_cats.category
LIMIT 0, 6