a.gtld-servers.NET has no NS records.
NS records for this zone are defined at gtld-servers.NET.
NS records for this zone are defined at gtld-servers.NET.
Question and response
Question
dig a.gtld-servers.NET. NS
Answer
The response has no answer records.
Authority
gtld-servers.NET. 86400 SOA av4.nstld.com. nstld.verisign-grs.com. 1771892308 3600 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0