FATAL 1:btree: BTP_CHAIN flag was expected (vacuum command)

When I try to vacuum, or vacuum analyze my database, I get the
following error:
FATAL 1:btree: BTP_CHAIN flag was expected

I've tried the following two commands:
/usr/local/pgsql/bin/psql -c "vacuum analyze;" billing
and
/usr/local/pgsql/bin/psql -c "vacuum;" billing

The top command used to work (for the last few years), so I assume
that something has changed with my data. Otherwise, the database
appears to be working ok.

Any pointers? I'm thinking about dumping all the data and
re-inserting it...
Jay Summet