Apex domains cannot have CNAME records.
Question and response
Question
dig co. NS
Answer
co. 5 NS d.registrydns.co.
co. 5 NS c.registrydns.co.
co. 5 NS a.registrydns.co.
co. 5 NS b.registrydns.co.
Authority
The response has no authority records.
Additional
a.registrydns.co. 168663 A 194.169.218.57
b.registrydns.co. 168663 A 185.24.64.57
c.registrydns.co. 168663 A 212.18.248.57
d.registrydns.co. 168663 A 212.18.249.57
a.registrydns.co. 168663 AAAA 2001:67c:13cc:0:0:0:1:57
b.registrydns.co. 168663 AAAA 2a04:2b00:13cc:0:0:0:1:57
c.registrydns.co. 168663 AAAA 2a04:2b00:13ee:0:0:0:0:57
d.registrydns.co. 168663 AAAA 2a04:2b00:13ff:0:0:0:0:57
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0