vacuumdb out of memory error

Hello all,

I run a nightly "vacuumdb -a -z" on my production server.  The output of
the command is emailed to me every night.  Today while checking my email
I received this:

vacuumdb: vacuuming database "postgres"
vacuumdb: vacuuming of database "postgres" failed: ERROR:  out of memory
DETAIL:  Failed on request of size 167772156.

Now this concerned me because I know that vacuum errors are often the
sign of data corruption, however the postgres database on my server has
no tables in it.  I logged into my server and reran the vacuumdb -a -z
command and it went though with no problem.  I also checked my log file
and it shows that there have been 25 out of memory errors on my server
today.

My question is:  Is this normal?  Should this make me nervous that I
have a problem?

Thanks,

Matt

ps: Some details:
I run autovacuum, but do the nightly vacuumdb as a sort of poor man's
maintenance window.
postgres=# select version();
                                                   version
-------------------------------------------------------------------------------------------------------------
 PostgreSQL 8.1.0 on i686-redhat-linux-gnu, compiled by GCC gcc (GCC)
3.3.3 20040412 (Red Hat Linux 3.3.3-7)