"broken trust chain" for non-existing AAAA records

[lst_hoe02 at kwsoft.de]

We are using Bind 9.7 at the border to resolve DNS queries for a small  
LAN. After moving forward in using IPv6 we discovered many "broken  
trust chain" errors in the bind log for non existing AAAA records. One  
example is

Nov 18 01:18:21 firewall named[27580]: error (broken trust chain)  
resolving 'smtp.g.comcast.net/AAAA/IN': 76.96.53.47#53
Nov 18 01:18:21 firewall named[27580]: error (broken trust chain)  
resolving 'smtp.g.comcast.net/AAAA/IN': 68.87.66.201#53
Nov 18 01:18:29 firewall named[27580]: error (broken trust chain)  
resolving 'smtp.g.comcast.net/AAAA/IN': 76.96.53.47#53
Nov 18 01:18:29 firewall named[27580]: error (broken trust chain)  
resolving 'smtp.g.comcast.net/AAAA/IN': 76.96.53.47#53

 From what i can see there is no DNSSEC for comcast.net so this should  
not happen and the A record just resolve fine. Any comment if this  
should worry me?

Regards

Andreas