Apex domains cannot have CNAME records.
Question and response
Question
dig ppp-bb.dion.ne.jp. NS
Answer
ppp-bb.dion.ne.jp. 5 NS dns16.dion.ne.jp.
ppp-bb.dion.ne.jp. 5 NS dns15.dion.ne.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0