int8 & primary key error

I received the following error:

pos=> CREATE TABLE item (upc INT8 CONSTRAINT pk_upc PRIMARY KEY);
NOTICE:  CREATE TABLE/PRIMARY KEY will create implicit index pk_upc for
table it
em
ERROR:  Can't find a default operator class for type 20.

anyone know what it means? I need to use an int8 field as the primary key in
a database I'm building but it won't let me... I have succesfully created
other tables so I know Postgres is working..
is there a place to look up error messages?

****************************************************************
Mike Erickson
Self-Proclaimed Expert at Everything
mikeisnot@home.com
www.camosun.bc.ca/~cst97023
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