BIND 9.2.3: complains about low TTL's on startup?

[Stefan Juon]

Hello, I'm actually migrating from BIND8 to BIND9 (9.2.3). I have some
entries with smaller TTL's (600) than the default TTL of the zone
(which is set to 43200).
Everytime I startup the named it complains like that:

Feb 17 17:55:56 xxx named[19917]: [ID 873579 daemon.warning]
dns_master_load: x.y.in-addr.arpa.db:753: TTL set to prior TTL (43200)

Sure, it's just a warning and the TTL is in fact on 600 as desired. So
does somebody know the reason of these warnings? Should I take care
about them?

Cheers, Stefan