Apex domains cannot have CNAME records.
Question and response
Question
dig 137.163.in-addr.arpa. NS
Answer
137.163.in-addr.arpa. 5 NS ns011-hgod4d2k6vl012gr.h.d-53.info.
137.163.in-addr.arpa. 5 NS ns011-hgod4d2k6vl012gr.h.d-53.net.
137.163.in-addr.arpa. 5 NS ns011-hgod4d2k6vl012gr.h.d-53.jp.
137.163.in-addr.arpa. 5 NS ns2.nttcom.ne.jp.
137.163.in-addr.arpa. 5 NS auth2.ns.gin.ntt.net.
137.163.in-addr.arpa. 5 NS auth1.ns.gin.ntt.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0