vfprintf %s NULL in "%s"

I'm running Monit to look at few services on OpenBSD 6.3 and I'm logging
to syslog.

In my /var/log/messages I routinely observe the following log entries:

Oct 27 22:00:01 alpha syslogd[97814]: restart
Oct 27 22:00:02 alpha monit: vfprintf %s NULL in "%s"
Oct 27 22:00:32 alpha last message repeated 11 times
Oct 27 22:02:32 alpha last message repeated 24 times
Oct 27 22:12:33 alpha last message repeated 120 times
Oct 27 22:22:33 alpha last message repeated 120 times
...and so on...

Monit is installed from ports.

$ monit --version
This is Monit version 5.25.1
Built with ssl, with ipv6, with compression, without pam and with large 
files

Does anybody know what does it mean? This log is not very useful...

Best regards,
Chris

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