ORA-01502 after drop partition

Hi all
I have a table that is partitioned based on month. The partitions are

200412
200501
200502
200503
200504
200505
200506
200507

I also have an index ('WIPMON.KFLOWF_INDEX1') that is based on the table. The index is not partitioned. Yesterday I dropped the partition 200412 from the table.
But when I performed an insert into the table today, I got the following error

ORA-01502: index 'WIPMON.KFLOWF_INDEX1' or partition of such index is in unusable state

What could be the cause of the problem? Must I rebuild the related indexes after I drop any partition?

yes, u have to rebuild

Thanks rommel
Is there any other way that does not require rebuild?

im afraid theres no other way

use local indexes - wont have this problem then

OK
I run the following SQL to create local indexes

Create index 'WIPMON.KFLOWF_INDEX1' on(.....) local;