Apex domains cannot have CNAME records.
Question and response
Question
dig 188.in-addr.arpa. NS
Answer
188.in-addr.arpa. 4 NS ns3.afrinic.net.
188.in-addr.arpa. 4 NS ns4.apnic.net.
188.in-addr.arpa. 4 NS pri.authdns.ripe.net.
188.in-addr.arpa. 4 NS rirns.arin.net.
188.in-addr.arpa. 4 NS ns3.lacnic.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0