On a recently installed machine, the Nagios check for mailq did not work. Instead it returned the following error:

This is on a QMail based machine running on FreeBSD. I compared the setup with an older FreeBSD machine, where the check works properly. This box was also running QMail but with an older set of Nagios plugins. The configuration was equal, except the check_mailq script itself, because it has evolved a little in the meantime.