NULL & NOT NULL

I'm (1) new to postgres & (2) trying to "convert" the sample db found
in "The Practical SQL Handbook" Bowman, et al. to postgres. When
trying to create the following table I get "parser: parse error at or
near 'null'"

create table authors (
au_id char(11) not null,
au_lname varchar(40) not null,
.
.
.
zip char(5) null
);

The table creation works fine twith the NOT NULL but won't work with
the NULL.
NULL is supported isn't it? I can't see where I'm making a syntax
error. Got me stumped. TIA.

........Bill Sneed Prospect, Maine......