Apex domains cannot have CNAME records.
Question and response
Question
dig 71.20.178.in-addr.arpa. NS
Answer
71.20.178.in-addr.arpa. 5 NS ns3.ovea.net.
71.20.178.in-addr.arpa. 5 NS ns2.ovea.eu.
71.20.178.in-addr.arpa. 5 NS ns1.ovea.fr.
71.20.178.in-addr.arpa. 5 NS ns4.ovea.ca.
Authority
The response has no authority records.
Additional
ns1.ovea.fr. 5 A 79.143.240.11
ns3.ovea.net. 5 A 79.143.240.13
ns4.ovea.ca. 5 A 79.143.245.182
ns1.ovea.fr. 5 AAAA 2a01:678:0:0:0:0:0:11
ns4.ovea.ca. 5 AAAA 2a01:67a:0:1:0:0:0:182
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0