表里存在NULL值导致 not in 子查询失效

--这是我第一次给业务写的SQL语句，查询来数据为0，当时没考虑太多就发出去了。 后业务给我看了几条测试数据，我发现确实查询结果不对。
--再看语句逻辑确实没错，后经过排查是BB里有几行cust_id为NULL导致not in 查询结果失效。

--改良前的语句
select * from AA a with(nolock)
where a.status_code='04'
and a.issue_date>='2012-01-01' and a.issue_date<='2012-10-30'
and LEFT(a.manorg_code,4)='8640'
and a.cust_id not in 
(
    select aa.cust_id from BB aa  with(nolock) where aa.sheet_type='Q' and aa.acttype_code in ('01','02','16','19')
)

--改良后的语句

select * from AA a with(nolock)
where a.status_code='04'
and a.issue_date>='2012-01-01' and a.issue_date<='2012-10-30'
and LEFT(a.manorg_code,4)='8640'
and a.cust_id not in 
(
select ISNULL(aa.cust_id,'') from BB aa with(nolock) where aa.sheet_type='Q' and aa.acttype_code in ('01','02','16','19')
)