关于SQL的问题，总是报错，但我不知道问题在哪里

SELECT a.gender,a.province,b.is_pay,COUNT(DISTINCTa.user_id)
case
when b.is_pay is null then '非付费用户',
when b.is_pay is not null then '付费用户'
END as 付费情况
FROM t_user_id_info as a
LEFT JOIN paid as b
on a.user_id=b.user_id
GROUP by a.province,a.gender

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'case
when b.is_pay is null then '非付费用户',
when b.is_pay is not null ' at line 2

COUNT(DISTINCTa.user_id) 后面少个逗号 ,