Un autoincrement en Oracle

[Bazzi]

Aqui esta lo que he intentado.

create table a1_source (
source_id number,
source_name data varchar2(100)
);

create sequence test_seq
start with 1
increment by 1
nomaxvalue;

create trigger test_trigger
before insert on a1_source
for each row
begin

select test_seq.nextval into :new.source_id from dual;

end;

El error es en: "select test_seq.nextval..."

An attempt was made to create a trigger on an object owned by SYS.
Do not create triggers on objects owned by SYS.
Me marca error en: "select test_seq.nextval" Ahi es donde me dice

gracias