Weird NOT IN effect with NULL values

When doing a subselect with NOT IN, as in

SELECT name
FROM customer
WHERE customer_id NOT IN (
SELECT customer_id
FROM salesorder
);

(from Bruce Momjian's book)

I get no rows if the result column returned by the subselect
contains NULL values. It works as expected if I remove the NULL values
from the result set. Is this behaviour correct and if so, why?

I am using 7.1 beta 4.

Regards, Frank