Increase ASH to more that 254m on 12c

Dear Friends

We are having the famous "Active Session History (ASH) performed an emergency flush" log and the current size of ASH and Emergency flush count is

select total_size,awr_flush_emergency_count from v$ash_info;

TOTAL_SIZE AWR_FLUSH_EMERGENCY_COUNT

---------- -------------------------

266338304                        30

As this is a oracle 12c installation and per 1385872.1, it is possible to increase the _ASH_SIZE to more that 254M. But when I try to set _ASH_SIZE to more that 254M I always get

alter system set "_ASH_SIZE"=512M

*

ERROR at line 1:

ORA-02097: parameter cannot be modified because specified value is invalid