FAIL: Stealth NS record leakage

When using a DNS Report tool at this site:

I get many “WARNINGS” and a few “FAIL”, one being the following on ALL my sites created via Interworx:

“Stealth NS record leakage”

"Your DNS servers leak stealth information in non-NS requests:

Stealth nameservers are leaked []!
Stealth nameservers are leaked []!

This can cause some serious problems (especially if there is a TTL discrepancy). If you must have stealth NS records (NS records listed at the authoritative DNS servers, but not the parent DNS servers), you should make sure that your DNS server does not leak the stealth NS records in response to other queries."