Apex domains cannot have CNAME records.
Question and response
Question
dig 128-255.56.217.74.in-addr.arpa. NS
Answer
128-255.56.217.74.in-addr.arpa. 5 NS ns-707.awsdns-24.net.
128-255.56.217.74.in-addr.arpa. 5 NS ns-1872.awsdns-42.co.uk.
128-255.56.217.74.in-addr.arpa. 5 NS ns-250.awsdns-31.com.
128-255.56.217.74.in-addr.arpa. 5 NS ns-1468.awsdns-55.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0