[Pdns-users] Multiple A records cause AXFR failure

[a b]

I added two A records, as follows:

SQL> insert into records(id, zone_id, fqdn, content, type) values(16, (select id from zones where name = 'dmz'), 'ntp.dmz', '172.16.2.2', 'A');

1 row created.

SQL> insert into records(id, zone_id, fqdn, content, type) values(66, (select id from zones where name = 'dmz'), 'ntp.dmz', '172.16.2.3', 'A');

1 row created.

SQL> update zones set serial = 2015112208 where name = 'dmz';

1 row updated.

SQL> commit;

Commit complete.

As soon as I do that, AXFR no longer works:

Nov 22 11:12:45 supermaster.domain.tld pdns[4849]: [ID 702911 local0.error] 1 domain for which we are master needs notifications
Nov 22 11:12:55 supermaster.domain.tld pdns[4849]: [ID 702911 local0.warning] Queued notification of domain 'dmz' to 172.16.2.5:53
Nov 22 11:12:56 supermaster.domain.tld pdns[4849]: [ID 702911 local0.error] AXFR of domain 'dmz' initiated by 172.16.2.5
Nov 22 11:12:56 supermaster.domain.tld pdns[4849]: [ID 702911 local0.error] TCP server is without backend connections in doAXFR, launching
Nov 22 11:12:56 supermaster.domain.tld pdns[4849]: [ID 702911 local0.warning] Removed from notification list: 'dmz' to 172.16.2.5:53 (was acknowledged)
Nov 22 11:12:56 supermaster.domain.tld pdns[4849]: [ID 702911 local0.warning] AXFR of domain 'dmz' allowed: client IP 172.16.2.5 is in per-domain ACL
Nov 22 11:12:56 supermaster.domain.tld pdns[4849]: [ID 702911 local0.error] TCP Connection Thread unable to answer a question because of a backend error, cycling
Nov 22 11:12:58 supermaster.domain.tld pdns[4849]: [ID 702911 local0.warning] No master domains need notifications

Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.warning] 1 slave domain needs checking, 0 queued for AXFR
Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.warning] Received serial number updates for 1 zones, had 0 timeouts
Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.warning] Domain 'dmz' is stale, master serial 2015112209, our serial 0
Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.error] Initiating transfer of 'dmz' from remote '172.16.2.4'
Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.error] AXFR started for 'dmz'
Nov 22 11:12:56 superslave.domain.tld pdns[3656]: [ID 702911 local0.error] Unable to AXFR zone 'dmz' from remote '172.16.2.4' (resolver): Remote nameserver closed TCP connection

...what do the errors "TCP Connection Thread unable to answer a question because of a backend error, cycling",
and
"Unable to AXFR zone 'dmz' from remote '172.16.2.4' (resolver): Remote nameserver closed TCP connection"
mean?

If I am doing something wrong, what is it?