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@ipa.fraunhofer.de
PostgreSQL version: 9.0.6
Operating system:   Windows Server 2007 SP2
Description:=20=20=20=20=20=20=20=20

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!