Apex domains cannot have CNAME records.
Question and response
Question
dig 45.in-addr.arpa. NS
Answer
45.in-addr.arpa. 5 NS arin.authdns.ripe.net.
45.in-addr.arpa. 5 NS u.arin.net.
45.in-addr.arpa. 5 NS r.arin.net.
45.in-addr.arpa. 5 NS x.arin.net.
45.in-addr.arpa. 5 NS y.arin.net.
45.in-addr.arpa. 5 NS z.arin.net.
Authority
The response has no authority records.
Additional
u.arin.net. 1 A 204.61.216.50
u.arin.net. 1 AAAA 2001:500:14:6050:ad:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0