Apex domains cannot have CNAME records.
Question and response
Question
dig vercel-dns.com. NS
Answer
vercel-dns.com. 5 NS ns1.vercel-dns.com.
vercel-dns.com. 5 NS ns2.vercel-dns.com.
Authority
The response has no authority records.
Additional
ns1.vercel-dns.com. 5 A 198.51.44.13
ns2.vercel-dns.com. 5 A 198.51.45.13
ns1.vercel-dns.com. 5 AAAA 2620:4d:4000:6259:7:13:0:1
ns2.vercel-dns.com. 5 AAAA 2a00:edc0:6259:7:13:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0