Apex domains cannot have CNAME records.
Question and response
Question
dig nttcom.ne.jp. NS
Answer
nttcom.ne.jp. 5 NS ns018-vu50aib6h0s4a6r1.v.d-53.net.
nttcom.ne.jp. 5 NS ns018-vu50aib6h0s4a6r1.v.d-53.info.
nttcom.ne.jp. 5 NS ns2.nttcom.ne.jp.
nttcom.ne.jp. 5 NS ns018-vu50aib6h0s4a6r1.v.d-53.jp.
Authority
The response has no authority records.
Additional
ns2.nttcom.ne.jp. 5 A 163.137.224.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0