null foreign key column

Hi all,

I would like to have a foreign key column in my table which allows null value also.
But the below create table sql command doesn't accept null value for person_id.
How to do in postgres?

create table event
(
event_id serial not null,
event_description char(255) ,
person_id serial ,
primary key (event_id),
foreign key (person_id)
references person (person_id)
);

regards
Jai