Apex domains cannot have CNAME records.
Question and response
Question
dig zc-dns.com. NS
Answer
zc-dns.com. 5 NS ns-a2.zc-dns.net.
zc-dns.com. 5 NS ns-a1.zc-dns.com.
zc-dns.com. 5 NS ns-a3.zc-dns.org.
Authority
The response has no authority records.
Additional
ns-a1.zc-dns.com. 5 A 157.7.33.88
ns-a2.zc-dns.net. 5 A 150.95.23.148
ns-a3.zc-dns.org. 5 A 150.95.19.148
ns-a1.zc-dns.com. 5 AAAA 2400:8500:3000:0:0:0:0:88
ns-a2.zc-dns.net. 5 AAAA 2404:8680:1101:310:0:0:0:53
ns-a3.zc-dns.org. 5 AAAA 2404:f080:1101:310:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0