What does "deleted from unreachable cache" mean?

[Peter Olsson]

Hello!

After my latest bind upgrade our slave server started
occasionally writing these messages to the log:

master 2a02:xxxx:xxxx:xxxx::2#53 (source ::#0) deleted from unreachable cache

master 62.xxx.xxx.2#53 (source 0.0.0.0#0) deleted from unreachable cache

DNS seems to work fine anyway, and all zonefiles in the slave
seem to update like they should, so everything seems ok. But I
would like to be certain that there is nothing to worry about,
so I wonder what these messages mean. (I didn't find anything
interesting in the list archives or in Google.)

Both master and slave are FreeBSD, running port bind97-9.7.6.1.

Thanks!

-- 
Peter Olsson                    pol at leissner.se