spamd-setup I/O error when run as cron job

[Scott Vanderbilt <lists_(_at_)_datagenic_(_dot_)_com>]

I am having some difficulties running spam-setup from cron. I am getting 
cron-daemon error emails containing the following:

   spamd-setup: Could not add blacklist relaydb-black: Input/output error

My crontab entry looks like this:

   0       *       *       *       *       /usr/libexec/spamd-setup

The offending spamd.conf entry looks like this:

   relaydb-black:\
         :black:\
         :msg=/home/fred/blacklistmsg.txt:\
         :method=exec:\
         :file=relaydb -lb:

I am able to successfully execute spamd-setup from the shell. Or, at least, 
I do not get any errors when I do so.

Does anyone know what the source of the I/O error when running as a cron 
job could be?

Many thanks in advance.