FATAL: root page 3 of "pg_class_oid_index" has level 0, expected 1

Hello,

I'm pretty new to pgsql, so speak slowly and draw plenty of pictures please
:P

I'm getting this error when I try to log into my database.

*FATAL: root page 3 of "pg_class_oid_index" has level 0, expected 1

*I've done some googleing and I found one mailing list item in another
language, and I couldn't decipher any useful information out of. I have no
idea what caused this or how it happened. The computer has been up for
months, but the database doesn't get very much use, just when I occasionally
use it with sql-ledger.

Thanks for your help,

Andrew