ORA-01918: user '' does not exist

Dear Colleague,

I have problem can't lock user, I have no idea what might happen. Could you give me a clue.

I can query to dba_users got the username but when every time i alter user <username> account lock; error is coming.

below is the example,

15:20:16 SQL> select username, account_status, lock_date, expiry_date, created, profile, password from dba_users

where15:20:18   2   username in ('app1','app2','app3','app4','app5');

USERNAME                       ACCOUNT_STATUS                   LOCK_DATE EXPIRY_DA CREATED   PROFILE  PASSWORD

------------------------------ -------------------------------- --------- --------- --------- -------- ------------------------------

app1                           OPEN                                                 02-JUN-14 DEFAULT

app2                           OPEN                                                 08-NOV-12 DEFAULT