Apex domains cannot have CNAME records.
Question and response
Question
dig ynwp.yahoo.co.jp. NS
Answer
ynwp.yahoo.co.jp. 4 NS c2.dns.ops.yahoo.co.jp.
ynwp.yahoo.co.jp. 4 NS c1.dns.ops.yahoo.co.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0