在查询某个月的登录用户数时有时会出现错误 ORA-01427: 单行子查询返回多个行，sql如下：

select NVL((select count(distinct user_id) as actnum

from (select user_id, to_char(login_date, 'YYYY-MM') riqi

from log_login where login_date between

to_date('2018-02-01', 'YYYY-MM-DD hh24:mi:ss') and

to_date('2018-02-28', 'YYYY-MM-DD hh24:mi:ss') and

user_oth_id is null

group by to_char(login_date, 'YYYY-MM'), user_id) t

group by t.riqi), 0) from dual

请问有谁知道报错原因吗，哪个子查询可能会返回多个行？