pga_aggregate_target

Hi All,
pga_aggregate_target has been incorrectly set to 21gb in one of our databases.

SQL> sho parameter pga

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
pga_aggregate_target                 big integer 21686M

When I try to change it to 1g or 800m I get the following error.

SQL>  alter system set pga_aggregate_target=1g scope=memory;
 alter system set pga_aggregate_target=1g scope=memory
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00068: invalid value 104850 for parameter _smm_max_size, must be between 0 and 0

Whereas when i try to set the scope to spfile it works