Unknown column 'd.usermodified' in 'on clause'

SELECT p.id,p.subject,p.modified,p.discussion,p.userid, d.name, d.timemodified, d.usermodified, d.groupid, u.firstname, u.lastname, u.email, u.picture , um.firstname AS umfirstname, um.lastname AS umlastname FROM mdl_forum_discussions d, mdl_forum_posts p, mdl_user u LEFT JOIN mdl_user um on (d.usermodified = um.id) WHERE d.forum = '17' AND p.discussion = d.id AND p.parent = 0 AND p.userid = u.id ORDER BY d.timemodified DESC