Apex domains cannot have CNAME records.
Question and response
Question
dig arpa. NS
Answer
arpa. 72756 NS f.ns.arpa.
arpa. 72756 NS d.ns.arpa.
arpa. 72756 NS m.ns.arpa.
arpa. 72756 NS c.ns.arpa.
arpa. 72756 NS a.ns.arpa.
arpa. 72756 NS h.ns.arpa.
arpa. 72756 NS b.ns.arpa.
arpa. 72756 NS g.ns.arpa.
arpa. 72756 NS k.ns.arpa.
arpa. 72756 NS e.ns.arpa.
arpa. 72756 NS i.ns.arpa.
arpa. 72756 NS l.ns.arpa.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0