Apex domains cannot have CNAME records.
Question and response
Question
dig 78.in-addr.arpa. NS
Answer
78.in-addr.arpa. 5 NS ns4.apnic.net.
78.in-addr.arpa. 5 NS ns3.afrinic.net.
78.in-addr.arpa. 5 NS rirns.arin.net.
78.in-addr.arpa. 5 NS pri.authdns.ripe.net.
78.in-addr.arpa. 5 NS ns3.lacnic.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0