vfprintf %s NULL in " tls=%s:%s"

[Scott Vanderbilt]

I'm noticing many occurrences of the following log message in 
/var/log/messages:

Apr  2 05:15:02 benedict smtpd: vfprintf %s NULL in " tls=%s:%s"
They come in pairs, and look to be associated with daily emails 
originating from the host itself, usually in connection with daily 
maintenance emails.

Does anyone know what's causing them? Should I be concerned, and/or what 
can I do to address their underlying cause?

Many thanks in advance.