Apex domains cannot have CNAME records.
Question and response
Question
dig 64h.57.29.125.in-addr.arpa. NS
Answer
64h.57.29.125.in-addr.arpa. 5 NS e.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS f.ns.cheetahmail.com.
64h.57.29.125.in-addr.arpa. 5 NS b.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS a.ns.cheetahmail.com.
64h.57.29.125.in-addr.arpa. 5 NS c.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS e.ns.cheetahmail.com.
64h.57.29.125.in-addr.arpa. 5 NS a.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS d.ns.cheetahmail.com.
64h.57.29.125.in-addr.arpa. 5 NS d.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS b.ns.cheetahmail.com.
64h.57.29.125.in-addr.arpa. 5 NS f.ns.64h.57.29.125.in-addr.arpa.
64h.57.29.125.in-addr.arpa. 5 NS c.ns.cheetahmail.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0