Apex domains cannot have CNAME records.
Question and response
Question
dig 231.125.42.in-addr.arpa. NS
Answer
231.125.42.in-addr.arpa. 4 NS ns010-513qsapg7nfrlocm.5.d-53.net.
231.125.42.in-addr.arpa. 4 NS ns010-513qsapg7nfrlocm.5.d-53.info.
231.125.42.in-addr.arpa. 4 NS ns010-513qsapg7nfrlocm.5.d-53.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0