Apex domains cannot have CNAME records.
Question and response
Question
dig support.iij.jp. NS
Answer
support.iij.jp. 5 NS ns003-q6jm39o7n2s04n93.q.d-53.jp.
support.iij.jp. 5 NS ns003-q6jm39o7n2s04n93.q.d-53.net.
support.iij.jp. 5 NS ns003-q6jm39o7n2s04n93.q.d-53.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0