Apex domains cannot have CNAME records.
Question and response
Question
dig rakus.co.jp. NS
Answer
rakus.co.jp. 5 NS ns-440.awsdns-55.com.
rakus.co.jp. 5 NS ns-1176.awsdns-19.org.
rakus.co.jp. 5 NS ns-850.awsdns-42.net.
rakus.co.jp. 5 NS ns-1881.awsdns-43.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0