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......