Getting error ORA-00846 when we increase MEMORY_TARGET

Hi,

We face some strange issue today as below :

SQL> ALTER SYSTEM SET MEMORY_TARGET = 8196M;
ALTER SYSTEM SET MEMORY_TARGET = 8196M
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00846: could not shrink MEMORY_TARGET to specified value

Enviroment details
Oracle version : 11.1.0.7

Parameter details :

memory_max_target                    big integer 16G
memory_target                        big integer 4G
pga_aggregate_target                 big integer 0
sga_target                           big integer 0
sga_max_size                         big integer 16G

Current SGA and PGA usage :

SQL> select * from V$MEMORY_DYNAMIC_COMPONENTS where current_size >0;

COMPONENT                      CURRENT_SIZE   MIN_SIZE   MAX_SIZE USER_SPECIFIED_SIZE OPER_COUNT LAST_OPER_TYP LAST_OPER LAST_OPER GRANULE_SIZE