plpgsql function problems

I would like to declare some non null variables
in plpgsql functions. When I do so, I always get
an error stating "null assignment to non null
variable" (in this sense). There is not even
an assignment obvious obvious to me:

ib=> CREATE FUNCTION something() RETURNS int4 AS 'DECLARE
ib'> i int4 not null DEFAULT 27;
ib'> BEGIN
ib'> return 0;
ib'> END;
ib'> ' LANGUAGE 'plpgsql';
CREATE
ib=> select something();
ERROR: NULL assignment to variable 'i' declared NOT NULL
ib=>

I really can't see the problem. Anybody of you?
(postgres 6.4.2)

(Maybe I should engage myself in the documentation project
if time allows, as according to 'obvious' documentation, this
seems to be fine...)

Thomas Mack
TU Braunschweig, Abt. Informationssysteme