Apex domains cannot have CNAME records.
Question and response
Question
dig 214.79.217.in-addr.arpa. NS
Answer
214.79.217.in-addr.arpa. 5 NS edns116.ultradns.org.
214.79.217.in-addr.arpa. 5 NS edns116.ultradns.com.
214.79.217.in-addr.arpa. 5 NS edns116.ultradns.biz.
214.79.217.in-addr.arpa. 5 NS edns116.ultradns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0