Apex domains cannot have CNAME records.
Question and response
Question
dig 32.252.169.210.in-addr.arpa. NS
Answer
32.252.169.210.in-addr.arpa. 3 NS ns-w2.mb.softbank.jp.
32.252.169.210.in-addr.arpa. 3 NS ns-e3.mb.softbank.jp.
Authority
The response has no authority records.
Additional
ns-e3.mb.softbank.jp. 2 A 210.175.70.247
ns-w2.mb.softbank.jp. 2 A 210.228.139.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0