Apex domains cannot have CNAME records.
Question and response
Question
dig ip4.gwtc.net. NS
Answer
ip4.gwtc.net. 2 NS ns2.gwtc.net.
ip4.gwtc.net. 2 NS ns1.gwtc.net.
Authority
The response has no authority records.
Additional
ns1.gwtc.net. 1 A 64.251.173.3
ns2.gwtc.net. 1 A 64.251.160.3
ns1.gwtc.net. 1 AAAA 2606:5700:f:1181:0:0:0:2
ns2.gwtc.net. 1 AAAA 2606:5700:f:1182:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0