Bug in pl/pgsql with hstore OR bug in pl/pgsql IF (text field is boolean?)

I can't get hstore value by key. i have very interesting error
DEMO SQL CODE:
------------------------------------------------------------------------------

ROLLBACK;
BEGIN;
-- VERSION INFO
SELECT VERSION();
-- RESULT:
-- PostgreSQL 8.3.4 on i686-redhat-linux-gnu, compiled by GCC gcc
(GCC) 4.1.2 20071124 (Red Hat 4.1.2-42)
-- TEST FOR HSTORE:
SELECT (('test' => 'testvalue')::hstore)->'test'; -- HSTORE WORKS!
-- HSTORE OK;

-- BUG REPRODUCTION FUNCTION
CREATE OR REPLACE FUNCTION "hstore_bug" () RETURNS pg_catalog.void AS
$body$
DECLARE
res HSTORE;
BEGIN
res := ('test' => NULL)::hstore;

IF res->'test' IS NULL THEN
RAISE EXCEPTION 'HSTORE IS OK';
END IF;

RAISE EXCEPTION 'HSTORE IS BUGGY';
END;
$body$
LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY DEFINER;

--- BUG REPRODUCTION:
SELECT hstore_bug();
-- ERROR MSG
-- ERROR: operator does not exist: hstore -> boolean
-- LINE 1: SELECT $1 ->'test' IS NULL
-- ^
-- HINT: No operator matches the given name and argument type(s). You
might need to add explicit type casts.
-- QUERY: SELECT $1 ->'test' IS NULL
-- CONTEXT: PL/pgSQL function "hstore_bug" line 6 at IF