How to use "not in" in MS Access?

Hi friends,
I have a MS Access database and i m working on two tables which include the fields Customer name and Booking date from CustomerMaster and PMReg table.
 
I want to display only those customer names who are not in PMReg table.
Note: In both table customer names exists.
 
My query is:

select a.customer from CustomerMaster as c,PMReg as p where a.active=p.active
and a.customer not in (select customer from PMReg)