pl/sql loop to enable user tracing

I am trying to use a loop to enable tracing of specific users with dbms_moniter and a pl/sql loop.  the procedure will create and compile but when I try to execute it it fails.
create or replace procedure trace_session
is
begin
 for rec in ( select sid, serial#
                    from   v$session
                    where  username in ('UserA','UserB')
  loop
    execute immediate 'dbms_monitor.session_trace_enable('||rec.sid||','||rec.serial#||')';
  end loop;
end;



It will fail with sys.trace_session invalid sql.

Is is better to use dbms_system.set_ev  instead?

thanks

any help is appreicated.