plpgsql: type of array cells

I'm trying to use an array of objects in plpgsql (postgresql 8.4):

drop type if exists test_t cascade;

create type test_t AS
(
i integer,
s text
);

create or replace function test2()
RETURNS SETOF test_t AS
$$
DECLARE
arr test_t ARRAY[3];
tmp test_t;
BEGIN
FOR i in 1 .. 3
LOOP
-- ok. Can write to test_t.i
tmp.i:=i;
-- ok. Can assign a cell from arr to a test_t object
arr[i]:=tmp;
-- error:
arr[i].i=3;
RETURN NEXT tmp;
END LOOP;
END;
$$
LANGUAGE plpgsql;

I'm getting an error when attempting to interpret this function:

NOTICE: drop cascades to function test2()
ERROR: syntax error at or near "."
LINE 21: arr[i].i=3;
^

********** Error **********

ERROR: syntax error at or near "."
SQL state: 42601
Character: 272

Isn't arr[i] of type test_t??

Thanks,
Amit