Apex domains cannot have CNAME records.
Question and response
Question
dig 9-13.29.208.82.in-addr.arpa. NS
Answer
9-13.29.208.82.in-addr.arpa. 5 NS ns.9-13.29.208.82.in-addr.arpa.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0