Apex domains cannot have CNAME records.
Question and response
Question
dig 128-25.25.30.115.in-addr.arpa. NS
Answer
128-25.25.30.115.in-addr.arpa. 5 NS dns.webcas.net.
128-25.25.30.115.in-addr.arpa. 5 NS ns.bit-isle.co.jp.
Authority
The response has no authority records.
Additional
ns.bit-isle.co.jp. 4 A 61.117.207.2
dns.webcas.net. 5 A 115.30.25.251
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0