Dangerous SQL usage of "NOT IN"

Here is example structure:

CREATE TABLE CUSTOMER
(
  ID                           INTEGER          NOT NULL,  
  FIRSTNAME                    VARCHAR          NOT NULL,
  LASTNAME                     VARCHAR          NOT NULL
);
CREATE TABLE CARD
(
  ID                          INTEGER           NOT NULL,
  CUSTOMERID                  INTEGER,
  SOMEDATA                    VARCHAR           DEFAULT 'ffff'
);
insert into card (id, customerid, somedata)
values (1, null, 'abcd');
insert into card (id, customerid, somedata)
values (2, null, 'efgh');
insert into card (id, customerid, somedata)
values (3, null, 'ijkl');
insert into customer (id, firstname, lastname)
values (1, 'John', 'Doe');
insert into customer (id, firstname, lastname)
values (2, 'Jane', 'Doe');
insert into customer (id, firstname, lastname)
values (3, 'Jim', 'Doe');
update card set customerid = 1 where id = 2;

SELECT
    MIN (customer.ID)
FROM customer
WHERE customer.ID NOT IN 
    (SELECT customerId FROM card WHERE customerId IS NOT NULL)

Last SELECT statement returns 2 which is ok but if you remove this part "WHERE customerId IS NOT NULL" all you get is NULL.
Database recognizes NULL value as "unknown".
It can't tell "I'm sure there IS NO customer.ID in that SELECT." because there are unknowns!!!
:)