Apex domains cannot have CNAME records.
Question and response
Question
dig acom.co.jp. NS
Answer
acom.co.jp. 5 NS ns-1013.awsdns-62.net.
acom.co.jp. 5 NS ns-1341.awsdns-39.org.
acom.co.jp. 5 NS ns-2027.awsdns-61.co.uk.
acom.co.jp. 5 NS ns-226.awsdns-28.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0