ORA-02097 / Parameter Cannot Be Modified Because Specified Value Is Invalid

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 smmmax_size, must be between 0 and 0

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

SQL> alter system set pga_aggregate_target=1g scope=spfile;
System altered.
Version is 11.2.0.3 on RHEL 5

how I can set the pga_aggregate_target right, I am not sure whether bouncing will work or not since i m doubtful that I may get the same error related to smmmax_size while bouncing with the new pga_aggregate_target.