为什么老是报Cannot SET SEVEROUTPUT解决方案

为什么老是报Cannot SET SEVEROUTPUT
SQL>   conn   system/manage   as   sysdba;
Connected   to   Oracle9i   Enterprise   Edition   Release   9.2.0.1.0  
Connected   as   SYS

SQL>   set   severoutput   on;
Cannot   SET   SEVEROUTPUT

望各位大虾们教教小弟

[解决办法]
set serveroutput on