Newbie: trigger with sequences

Is there anyway to create a trigger that will automatically populate the primary key field with the NEXTVAL from the sequence for that table? I tried something like this (with sequence emp_ids already created):

CREATE OR REPLACE trigger EMP_TRIG
before insert on EMP
for each row
begin
 :new.empno := emp_ids.NEXTVAL;
end EMP_TRIG;
/

but it returns with an error that "Sequence reference 'EMP_IDS.NEXTVAL' not allowed in this context".

Thanks.

--
Nikki