error during vacuum full

When trying to perform a full vacuum I am getting the following error:

ERROR:  No one parent tuple was found

Plain vacuum works fine.  Thinking it might be a problem with the 
indexes I have rebuilt them but still get the error.  What does this 
error indicate and what are my options to solve the problem?

I am running:  PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 
2.96 on a RedHat 7.3 system.

thanks,
--Barry