[CentOS] Zone transfers not working for subdomain

I have a domain, let's call it "example.com".  I am able to do zone
transfers on the local host as follows:

dig example.com AXFR @localhost

This command outputs all of the contents of the zone as expected.  I
am unable to do zone transfers on my subdomain though:

dig subdomain.example.com AXFR @localhost

; <<>> DiG 9.2.1 <<>> subdomain.example.com AXFR @localhost
;; global options:  printcmd
; Transfer failed.

Below are my config files, truncated and with host names changed to
protect the innocent:

**** From /var/named/zones.conf ***
****************************************************
zone "example.com" {
        type master;
        file "master/example.com";
};

zone "subdomain.example.com" {
        type master;
        file "master/subdomain.example.com";
};



*** From /var/named/master/example.com ***
****************************************************
@       IN      SOA     ns.example.com. root.example.com. (
                        2008092902
                        1H
                        10M
                        9D
                        5M )

                        NS      ns1.slaveserver.net.
                        NS      ns2.slaveserver.net.
                        A       66.45.102.50
(whole bunch of CNAME and A records removed for brevity's sake)



*** From /var/named/master/subdomain.example.com ***
****************************************************

$ttl 38400
@       IN      SOA     ns.example.com. root.example.com. (
                        2008092222
                        5M
                        5M
                        7D
                        38400 )

subdomain.example.com  IN      NS        host1.internal.ops.
subdomain.example.com   IN      NS       host2.internal.ops.
(Note that internal.ops is for internal use only)


What am I missing here?