Apex domains cannot have CNAME records.
Question and response
Question
dig isc.org. NS
Answer
isc.org. 5 NS ns1.isc.org.
isc.org. 5 NS nsp.dnsnode.net.
isc.org. 5 NS ns3.isc.org.
isc.org. 5 NS ns.isc.afilias-nst.info.
isc.org. 5 NS ns2.isc.org.
Authority
The response has no authority records.
Additional
ns1.isc.org. 5 A 149.20.2.26
ns2.isc.org. 5 A 199.6.1.52
ns3.isc.org. 5 A 51.75.79.143
ns1.isc.org. 5 AAAA 2001:500:6b:2:0:0:0:26
ns2.isc.org. 5 AAAA 2001:500:60:d:0:0:0:52
ns3.isc.org. 5 AAAA 2001:41d0:701:1100:0:0:0:2c92
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0