Apex domains cannot have CNAME records.
Question and response
Question
dig synm.jp. NS
Answer
synm.jp. 5 NS ns-1838.awsdns-37.co.uk.
synm.jp. 5 NS ns-43.awsdns-05.com.
synm.jp. 5 NS ns-991.awsdns-59.net.
synm.jp. 5 NS ns-1389.awsdns-45.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0