Apex domains cannot have CNAME records.
Question and response
Question
dig 180.51.209.in-addr.arpa. NS
Answer
180.51.209.in-addr.arpa. 5 NS ns1.he.net.
180.51.209.in-addr.arpa. 5 NS ns3.he.net.
180.51.209.in-addr.arpa. 5 NS ns2.he.net.
180.51.209.in-addr.arpa. 5 NS ns5.he.net.
180.51.209.in-addr.arpa. 5 NS ns4.he.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0