Daily test signer.ods_signer.serial fails on freebsd9 and freebsd10 machines

Backtrace for the core dump created by ods-signerd on freebsd9:

#0 0x0000000801dcc23c in thr_kill () from /lib/libc.so.7
#1 0x0000000801e7063b in abort () from /lib/libc.so.7
#2 0x000000000043e266 in ods_fatal_exit (format=<value optimized out>)
at ../../common/log.c:412
#3 0x000000000041a7ab in ixfr_del_rr (ixfr=0x8025868e0, rr=0x802e8bb20)
at ../../../signer/src/signer/ixfr.c:145
#4 0x000000000041edfd in rrset_diff (rrset=0x802582800, is_ixfr=0,
more_coming=0) at ../../../signer/src/signer/rrset.c:385
#5 0x0000000000421d0d in zone_update_serial (zone=0x802484480)
at ../../../signer/src/signer/zone.c:546
#6 0x0000000000412dc5 in worker_work (worker=0x8025a9040)
at ../../../signer/src/daemon/worker.c:367
#7 0x00000000004137ba in worker_start (worker=0x8025a9040)
at ../../../signer/src/daemon/worker.c:754
#8 0x000000000040f821 in worker_thread_start (arg=0x8025a9040)
at ../../../signer/src/daemon/engine.c:348
#9 0x0000000801b4ed34 in pthread_getprio () from /lib/libthr.so.3
#10 0x0000000000000000 in ?? ()

The syslog reads:

Jul 14 09:17:33 freebsd9-ods06 ods-signerd: [engine] signer started (version 2.1.0-dev), pid 41000
Jul 14 09:17:33 freebsd9-ods06 ods-signerd: [signconf] zone ods signconf: RESIGN[P2M] REFRESH[P3M] VALIDITY[P7M] DENIAL[P7M] KEYSET[PT0S] JITTER[PT12H] OFFSET[PT5M] NSEC[50] DNSKEYTTL[P3M] SOATTL[P3M] MINIMUM[P14D] SERIAL[counter]
Jul 14 09:17:33 freebsd9-ods06 ods-signerd: [STATS] ods 1001 RR[count=263 time=0(sec)] NSEC3[count=24 time=0(sec)] RRSIG[new=51 reused=0 time=0(sec) avg=0(sig/sec)] TOTAL[time=0(sec)]
Jul 14 09:17:39 freebsd9-ods06 ods-signerd: [namedb] zone ods enforcing serial 2000
Jul 14 09:17:39 freebsd9-ods06 ods-signerd: [STATS] ods 2000 RR[count=1 time=0(sec)] NSEC3[count=0 time=0(sec)] RRSIG[new=1 reused=50 time=0(sec) avg=0(sig/sec)] TOTAL[time=0(sec)]
Jul 14 09:17:44 freebsd9-ods06 ods-signerd: ../../../signer/src/signer/ixfr.c:145: ixfr_del_rr: assertion !ixfr->part[0]->soamin failed
Jul 14 09:17:45 freebsd9-ods06 ods-enforcerd: [engine] enforcer shutdown
Jul 14 09:17:45 freebsd9-ods06 ods-enforcerd: [engine] enforcerd (pid: 40986) stopped with exitcode 0