Apex domains cannot have CNAME records.
Question and response
Question
dig ambisys.net. NS
Answer
ambisys.net. 4 NS ns1.ambisys.jp.
ambisys.net. 4 NS ns2.ambisys.jp.
ambisys.net. 4 NS ns3.ambisys.jp.
Authority
The response has no authority records.
Additional
ns1.ambisys.jp. 4 A 210.175.226.118
ns2.ambisys.jp. 4 A 210.250.61.5
ns3.ambisys.jp. 4 A 61.215.192.140
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0