BUG #6400: function arguments not accepted

The following bug has been logged on the website:

Bug reference: 6400
Logged by: Sandra Knoch
Email address: knoch(at)ipa(dot)fraunhofer(dot)de
PostgreSQL version: 9.0.6
Operating system: Windows Server 2007 SP2
Description:

I have a strange problem regarding functions with parameters. The parameters
are not accepted and I always get the error column "user" does not exist,
where user is the parameter name and not a column. The function looks like
this:

CREATE OR REPLACE FUNCTION tissue.insert_biopsy_data
(
"user" integer,
in_date date,
in_description varchar,
in_weight numeric,
in_size numeric
)
RETURNS integer AS
$$
declare
result integer;
begin
INSERT INTO tissue.biopsy (id_biopsy, f_user,introduction_date,
description, weight, size)
VALUES
(
DEFAULT,
user,
in_date,
in_description,
in_weight,
in_size
);
SELECT id_biopsy FROM tissue.biopsy INTO result;
Return result;
end
$$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

It works with my old PostgreSQL 8.4 version, but not with the new one. What
is the problem here? Why is the argument not recognized as argument but as a
column name? Thank you for any help!