Apex domains cannot have CNAME records.
Question and response
Question
dig iris.arpa. NS
Answer
iris.arpa. 4 NS c.iana-servers.net.
iris.arpa. 4 NS ns3.lacnic.net.
iris.arpa. 4 NS a.iana-servers.net.
iris.arpa. 4 NS b.iana-servers.net.
iris.arpa. 4 NS ns4.apnic.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0