ORA-01940: cannot drop a user that is currently connected

SQL>  drop user SID;
 drop user SID
*
ERROR at line 1:
ORA-01940: cannot drop a user that is currently connected


SQL> select sid,serial# from v$session where username = 'SID';

       SID    SERIAL#
---------- ----------
       125       2058

SQL> ALTER SYSTEM KILL SESSION '125,2058#';
ALTER SYSTEM KILL SESSION '125,2058#'
*
ERROR at line 1:
ORA-00026: missing or invalid session ID


SQL> ALTER SYSTEM KILL SESSION '125,2058';

System altered.

SQL> select sid,serial# from v$session where username = 'SID';

       SID    SERIAL#
---------- ----------
       125       2058

SQL> drop user SID;

User dropped.