With inline-signing: Log error "receive_secure_serial: unchanged"

Summary

With inline-signing enabled, I see an error like this in the logs on every (re)load of bind:

zone XXX.de/IN (signed): receive_secure_serial: unchanged

The message itself does not indicate an error, but it is sent with error severity and hence rendered reg in the systemd journal.

Steps to reproduce

Have an inline-signing zone, and reload bind.

What is the current bug behavior?

A message like the above shows up in the system log, and it is marked with the error severity.

What is the expected correct behavior?

Either the message should say more clearly what the problem is, or the message severity should be reduced (most of the other messages on a zone reload seem to be "notice" or so).

This is with bind9 "1:9.10.3.dfsg.P4-12.3+deb9u4" on Debian stretch.