Apex domains cannot have CNAME records.
Question and response
Question
dig 160.153.153.in-addr.arpa. NS
Answer
160.153.153.in-addr.arpa. 5 NS dns901-1.jp1.ecl.ntt.com.
160.153.153.in-addr.arpa. 5 NS dns901-4.sg1.ecl.ntt.com.
160.153.153.in-addr.arpa. 5 NS dns901-3.us1.ecl.ntt.com.
160.153.153.in-addr.arpa. 5 NS dns901-2.uk1.ecl.ntt.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0