Apex domains cannot have CNAME records.
Question and response
Question
dig servb.jp. NS
Answer
servb.jp. 4 NS ns-788.awsdns-34.net.
servb.jp. 4 NS ns-233.awsdns-29.com.
servb.jp. 4 NS ns-2012.awsdns-59.co.uk.
servb.jp. 4 NS ns-1032.awsdns-01.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0