Apex domains cannot have CNAME records.
Question and response
Question
dig nudns.com. NS
Answer
nudns.com. 5 NS ns4.nudns.com.
nudns.com. 5 NS ns1.nudns.com.
nudns.com. 5 NS ns2.nudns.com.
nudns.com. 5 NS ns3.nudns.com.
Authority
The response has no authority records.
Additional
ns1.nudns.com. 5 A 199.167.66.1
ns2.nudns.com. 5 A 104.37.178.1
ns3.nudns.com. 5 A 199.167.66.2
ns4.nudns.com. 5 A 104.37.178.2
ns1.nudns.com. 5 AAAA 2610:1c8:b001:0:0:0:0:1
ns2.nudns.com. 5 AAAA 2610:1c8:b002:0:0:0:0:1
ns3.nudns.com. 5 AAAA 2610:1c8:b001:0:0:0:0:2
ns4.nudns.com. 5 AAAA 2610:1c8:b002:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0