Default crontab location not working for root

I'm using 10.04.3 LTS with default cfengine3 installation (3.0.2+dfsg-2)

When executing cf-agent from command line as root, I get this message:

> File /var/spool/cron/crontabs/root was marked for editing but could not be opened

To get around this, I edited the following into /var/lib/cfengine3/site.cf

bundle common g {
   vars:
      Ubuntu::
          "crontab" string => "/etc/crontab";
}