please explain error: expected covering NSEC3, got an exact match

[Nate Itkin]

Sorry about that truncated subject line. Let's try that again.

If someone would kindly explain what this error message means, I would
appreciate it.  I'm running BIND 9.6.2-P1 and I get quite a few of these:

28-Mar-2010 21:02:27.467 dnssec: warning: client 200.160.7.134#6363: view external: expected covering NSEC3, got an exact match

Thank you,
Nate Itkin