Apex domains cannot have CNAME records.
Question and response
Question
dig 159.in-addr.arpa. NS
Answer
159.in-addr.arpa. 5 NS z.arin.net.
159.in-addr.arpa. 5 NS x.arin.net.
159.in-addr.arpa. 5 NS u.arin.net.
159.in-addr.arpa. 5 NS r.arin.net.
159.in-addr.arpa. 5 NS y.arin.net.
159.in-addr.arpa. 5 NS arin.authdns.ripe.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0