Error: Illegal mix of collations (utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,IMPLICIT) for operation 'if'

SELECT t1.*, IF (t1.`account_id` > 0, t2.`username`, t1.`author`) author FROM `v2206_guestbook` t1 LEFT JOIN `v2206_accounts` t2 ON t1.`account_id` = t2.`id` WHERE t1.`status` = 'active' ORDER BY t1.`date` DESC LIMIT 0, 20