Unable to insert partition

I'm on Oracle 10204 and we have a partitioned table. I'm trying to create a partition that is immediately less than maxvalue but I get ORA-14074 error. Why I'm not able to create the partition below? How can I add the June partiiton.? We have to create new partition every month and we will retain only 6 months of data after that we will keep purging the partitions

SQL> select partition_name,HIGH_VALUE
  2  from DBA_TAB_PARTITIONS
  3  where table_name='TRANSAC_DETAIL_HSI_WEB_PART' order by 1;

PARTITION_NAME                 HIGH_VALUE
------------------------------ --------------------------------------------------------------------------------
P201112                        TO_DATE(' 2012-01-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIA