Apex domains cannot have CNAME records.
Question and response
Question
dig 128-27.166.24.198.in-addr.arpa. NS
Answer
128-27.166.24.198.in-addr.arpa. 5 NS ptrdns2.registrar-servers.com.
128-27.166.24.198.in-addr.arpa. 5 NS ptrdns3.registrar-servers.com.
128-27.166.24.198.in-addr.arpa. 5 NS ptrdns4.registrar-servers.com.
128-27.166.24.198.in-addr.arpa. 5 NS ptrdns1.registrar-servers.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0