Apex domains cannot have CNAME records.
Question and response
Question
dig 128-25.171.148.180.in-addr.arpa. NS
Answer
128-25.171.148.180.in-addr.arpa. 5 NS vd.bit-isle.co.jp.
128-25.171.148.180.in-addr.arpa. 5 NS ns3.bit-isle.co.jp.
Authority
The response has no authority records.
Additional
ns3.bit-isle.co.jp. 5 A 202.131.202.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0