ORA-01031: insufficient privileges

That error has been displayed through this scenario:
SQL> select username, account_status from dba_users

USERNAME                                               ACCOUNT_STATUS
-----------------------------------------------------------------------------------------
SCOTT                                                      EXPIRED & LOCKED
XS$NULL                                                  EXPIRED & LOCKED

SQL> alter user scott identified by scott account unlock;

User altered.

SQL> alter user xs$null identified by xs$null account unlock;
alter user xs$null identified by xs$null account unlock
                                                 *
ERROR at line 1:
ORA-01031: insufficient privileges

SQL>

please help me with this matter thanks in advance! :P

Regards,
Nuggets_05