Apex domains cannot have CNAME records.
Question and response
Question
dig tld.md. NS
Answer
tld.md. 5 NS a.tld.md.
tld.md. 5 NS f.tld.md.
tld.md. 5 NS c.tld.md.
tld.md. 5 NS b.tld.md.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0