Oracle Database Discussions

Hi,
I'm try to change the parallel_max_servers parameter from 5 to 2 with the command below but it fails with the error ORA-02095. Could someone tell me why this error is occurring and how too change this parameter?

SQL> alter system set parallel_max_servers=2 scope=both;
alter system set parallel_max_servers=2 scope=both
*
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified

Any help will be appreciated.

Steve