Apex domains cannot have CNAME records.
Question and response
Question
dig mpub.jp. NS
Answer
mpub.jp. 5 NS ns-1561.awsdns-03.co.uk.
mpub.jp. 5 NS ns-264.awsdns-33.com.
mpub.jp. 5 NS ns-660.awsdns-18.net.
mpub.jp. 5 NS ns-1192.awsdns-21.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0