Apex domains cannot have CNAME records.
Question and response
Question
dig 5.in-addr.arpa. NS
Answer
5.in-addr.arpa. 5 NS ns3.afrinic.net.
5.in-addr.arpa. 5 NS ns4.apnic.net.
5.in-addr.arpa. 5 NS rirns.arin.net.
5.in-addr.arpa. 5 NS ns3.lacnic.net.
5.in-addr.arpa. 5 NS pri.authdns.ripe.net.
Authority
The response has no authority records.
Additional
ns3.lacnic.net. 4 A 200.3.13.14
ns3.afrinic.net. 4 A 204.61.216.100
ns4.apnic.net. 4 A 202.12.31.53
pri.authdns.ripe.net. 4 A 193.0.9.5
rirns.arin.net. 4 A 199.253.249.53
ns3.lacnic.net. 4 AAAA 2001:13c7:7002:3000:0:0:0:14
ns3.afrinic.net. 4 AAAA 2001:500:14:6100:ad:0:0:1
ns4.apnic.net. 4 AAAA 2001:dd8:12:0:0:0:0:53
pri.authdns.ripe.net. 4 AAAA 2a13:27c0:30:0:0:0:0:5
rirns.arin.net. 4 AAAA 2620:38:2000:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0