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)