PL/pgSQL: "record not assigned yet" meaning?

I have a plpgsql function that sets a column value on insert if it was not
supplied:

CREATE FUNCTION set_expense_unit()
RETURNS OPAQUE AS
'DECLARE
t RECORD;
BEGIN
if NEW.expense_unit_id isnull then
SELECT INTO t eu.*
FROM expense_unit eu, expense_type et
WHERE et.id = NEW.expense_type_id
AND et.expense_unit_id = eu.id;

NEW.expense_unit_id := t.id;
end if;

RETURN NEW;
END;'
LANGUAGE 'plpgsql';

DROP TRIGGER expense_insert_trigger ON expense;
CREATE TRIGGER expense_insert_trigger
BEFORE INSERT ON expense
FOR EACH ROW
EXECUTE PROCEDURE set_expense_unit();

Each time it runs, I get this error:

ERROR: record t is unassigned yet

Any ideas what this means?

Regards,
Ed Loehr