Apex domains cannot have CNAME records.
Question and response
Question
dig 27-160.253.174.188.in-addr.arpa. NS
Answer
27-160.253.174.188.in-addr.arpa. 4 NS ns2.he.net.
27-160.253.174.188.in-addr.arpa. 4 NS ns1.he.net.
27-160.253.174.188.in-addr.arpa. 4 NS ns4.he.net.
27-160.253.174.188.in-addr.arpa. 4 NS ns5.he.net.
27-160.253.174.188.in-addr.arpa. 4 NS a.ns.madduck.net.
27-160.253.174.188.in-addr.arpa. 4 NS ns3.he.net.
Authority
The response has no authority records.
Additional
a.ns.madduck.net. 4 A 188.174.253.164
a.ns.madduck.net. 4 AAAA 2001:a60:902f:0:0:53:0:0
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0