"can't validate existing negative responses (not a zone cut)" messages

[Chris Thompson]

Since upgrading our main recursive nameservers to BIND 9.7.2-P2 (and
using a trust anchor for the root and lookaside via dlv.isc.org) I am
seeing a scatter of warning messages like this:

Oct  1 19:47:19 dnssec: warning: validating @1c29d580:
  115.197.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)
Oct  1 19:47:19 dnssec: warning: validating @8d34550:
  115.197.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)
Oct  2 11:25:28 dnssec: warning: validating @11d37f18:
  32.197.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)
Oct  3 01:39:21  dnssec: warning: validating @160f4260:
  205.205.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)
Oct  3 08:40:14  dnssec: warning: validating @12cd35c0:
  20.204.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)
Oct  3 16:53:10 dnssec: warning: validating @14c9cd70:
  98.206.101.95.IN-ADDR.ARPA PTR:
  can't validate existing negative responses (not a zone cut)

What do they mean, exactly? And should I be worrying about them?
They all seem to refer to PTR records (not all of them for IP
addresses in 95.101/16, but many of them are).

-- 
Chris Thompson
Email: cet1 at cam.ac.uk