primary key question

create table aaa(
field1 .... not null,
field2 ....,
....,
primary key (field1, field2)
);

Based on the above table definition, field2 allows null values. But
after the table created based on the above script, field2 becomes not
null. The only conclusion I come up with is setting the field as part of
the primary key, PostgreSQL automatically sets the field to not null. Or
is it something else?

Is this a feature or bug?