Error when assigning default value for function parameter

I'm a bit in the dark:
I use Windows XP
I use pgAdmin ver 1.8.4
I use PostGres 8.3.6

All the documentation points out that it is allowed to assign default
values to parameters, but I get a syntax error here.

I would appreciate feedback.

CREATE OR REPLACE FUNCTION myfunc(a integer , b integer DEFAULT 100)
RETURNS INTEGER AS $$
    select  200;
 $$ language sql;

ERROR:  syntax error at or near "DEFAULT"
LINE 1: ...TE OR REPLACE FUNCTION ___myfunc(a int, b integer DEFAULT
10...