Cannot read block 0

I have a very small table called stdheader that contains 2 columns, the
first is an ID (int4) and the other is a text field. Periodically, the
data in the table is refreshed by deleting all of the existing records
and reloading the new data from a tab-delimited text file. The data
appeared to load fine, but this time when I do a select * from
stdheader; within psql, I get the following message:

ERROR: cannot read block 0 of stdheader
Failed Assertion("!(((*&(buf->io_in_progress_lock)) == 0)):", File:
"bufmgr.c", Line: 665)
!(((*&(buf->io_in_progress_lock)) == 0)) (0) [Illegal seek]
NOTICE: Message from PostgreSQL backend:
The Postmaster has informed me that some other backend died
abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am going to
terminate
your database system connection and exit.
Please reconnect to the database system and repeat your query.

When I try to drop the table using:
drop table stdheader;

I get the message: broken pipe

Any clues? We are running version 6.3.1 of postgresql.

Thanks for any assistance

Toni Steinhauer