Trouble loading a zone file after updating BIND

[Mike Diggins]

I have a "hidden" master name server for the domain domain.CA. The name 
servers for this domain are in the subdomain sub.Domain.CA as ns1 and ns2. 
Those are the name servers my clients query. The zone file starts off like 
this:

$TTL 3600
Domain.CA.  IN  SOA  ns1.sub.Domain.CA.  dnsgrp.Domain.CA. (
                         20782   ; Serial
                         3600    ; Refresh
                         1800    ; Retry
                         604800  ; Expire
                         3600 )  ; Negative TTL
;
;------------------------------------------------------------------------------
;                                Name Servers
;------------------------------------------------------------------------------
                         IN  NS  ns1.sub.Domain.CA.
                         IN  NS  ns2.sub.Domain.CA.
...etc

On the master name server, I'm upgrading BIND from an older version, 
9.2.1, to 9.7. However, when I attempt to load this zone Domain.CA, it 
gives me an error:

Mar 29 09:52:13 ns-master named: zone Domain.CA/IN: NS 'ns2.sub.Domain.CA' 
has no address records (A or AAAA)
Mar 29 09:52:13 ns-master named: zone Domain.CA/IN: NS 'ns1.sub.Domain.CA' 
has no address records (A or AAAA)
Mar 29 09:52:13 ns-master named: zone Domain.CA/IN: not loaded due to 
errors.

The A records for the two nameservers exist in the sub.Domain.CA zone 
file. I can fix the error by adding the two nameserver A records to the 
Domain.CA zone file but I'm wondering why this is an error with 9.7, and 
not 9.2.1, and is this the correct way to fix this?

I hope this isn't DNS 101 ;)

-Mike