BUG #2412: Foreing key accept nulls

The following bug has been logged online:

Bug reference:      2412
Logged by:          Sidar Lopez Cruz
Email address:      sidarlopez@hotmail.com
PostgreSQL version: 8.1.3
Operating system:   Ubuntu 6.04
Description:        Foreing key accept nulls
Details:

I think that when I create table that have a foreing key to another one
these FK may not accept nulls by default.