Oracle Failed Login Trigger - Capture Oracle User

How can the Oracle username be captured using a trigger and table for a failed log on attempt.  I'm aware Oracle auditing captures this information. See example below:

create table stats$user_log
(
   terminal            varchar2(255),
   os_username         varchar2(255),
   username            varchar2(30),
   extended_timestamp  date)
tablespace users
;

CREATE OR REPLACE TRIGGER failed_logon_trg
  AFTER SERVERERROR ON DATABASE
  when (ora_server_error(1)=1017)
BEGIN
   insert into stats$user_log
      (terminal, os_username, username, extended_timestamp)
   values
      (SYS_CONTEXT('USERENV','TERMINAL'),
      SYS_CONTEXT('USERENV','OS_USER'),
      USER,
      sysdate);
      COMMIT;
END failed_logon_trg;
/

I've tried both 1) USER result NULL and 2) SYS_CONTEXT('USERENV','CURRENT_USER') result SYS