Apex domains cannot have CNAME records.
Question and response
Question
dig 91.138.210.xf.2iij.net. NS
Answer
91.138.210.xf.2iij.net. 5 NS rev.iij-dns.net.
91.138.210.xf.2iij.net. 5 NS rev.iij-dns.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0