Apex domains cannot have CNAME records.
Question and response
Question
dig 219.29.81.in-addr.arpa. NS
Answer
219.29.81.in-addr.arpa. 5 NS edns116.ultradns.biz.
219.29.81.in-addr.arpa. 5 NS edns116.ultradns.com.
219.29.81.in-addr.arpa. 5 NS edns116.ultradns.net.
219.29.81.in-addr.arpa. 5 NS edns116.ultradns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0