a serious error in DNS configurations, yet a (too) common one
Condition when DNS servers listed in a zone are not configured to be authoritative for the zone.
Listing a host (name server) that does not contain a Start of Authority (SOA) record for a domain name on a Domain Name Registration Agreement. [see also: Start of Authority (SOA) Record, Domain Name Registration Agreement, Host (Name Server)
This occurs when DNS servers listed in a zone were not configured to be authoritative for the zone.