a.gtld-servers.net has no CNAME records.
Question and response
Question
dig a.gtld-servers.net. NS
Answer
The response has no answer records.
Authority
gtld-servers.NET. 8348 SOA av4.nstld.com. nstld.verisign-grs.com. 1772669909 3600 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0