What does this mean ? INSIST(zone->type == dns_zone_stub) failed

[蔡火胜]

07-Dec-2011 22:55:29.457 zone.c:9655: INSIST(zone->type == 
dns_zone_stub) failed, back trace
07-Dec-2011 22:55:29.457 #0 0x4134eb in assertion_failed()+0x4b
07-Dec-2011 22:55:29.457 #1 0x569eca in isc_assertion_failed()+0xa
07-Dec-2011 22:55:29.457 #2 0x53b2f9 in refresh_callback()+0x1389
07-Dec-2011 22:55:29.457 #3 0x58481c in run()+0x1dc
07-Dec-2011 22:55:29.457 #4 0x399f2061b5 in _fini()+0x399ec6e9ed
07-Dec-2011 22:55:29.457 #5 0x399e6cd39d in _fini()+0x399e135bd5
07-Dec-2011 22:55:29.457 exiting (due to assertion failure)