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