Apex domains cannot have CNAME records.
Question and response
Question
dig 128.57.160.210.in-addr.arpa. NS
Answer
128.57.160.210.in-addr.arpa. 5 NS os12-dnsic1-dns1.anser.ne.jp.
128.57.160.210.in-addr.arpa. 5 NS ns6-tk02.ocn.ad.jp.
Authority
The response has no authority records.
Additional
os12-dnsic1-dns1.anser.ne.jp. 5 A 210.160.57.250
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0