ORA-00027

In the SQL*Plus session below, I connect as SYS to an Oracle 10 database. Then I show that there is only one user logged on as SYS i.e. me. I note the sid and serial# and try to use them to kill my current session. Oracle does not allow this and displays an ORA-00027:

SQL> conn / as sysdba
Connected.
SQL> show user
USER is "SYS"
SQL> select sid, serial# from v$session
  2  where username = 'SYS'
  3  /

       SID    SERIAL#
---------- ----------
       159          5

SQL> alter system kill session '159,5';
alter system kill session '159,5'
*
ERROR at line 1:
ORA-00027: cannot kill current session

SQL>