Apex domains cannot have CNAME records.
Question and response
Question
dig 157.128.in-addr.arpa. NS
Answer
157.128.in-addr.arpa. 5 NS a12-64.akam.net.
157.128.in-addr.arpa. 5 NS a5-66.akam.net.
157.128.in-addr.arpa. 5 NS a8-66.akam.net.
157.128.in-addr.arpa. 5 NS a1-32.akam.net.
157.128.in-addr.arpa. 5 NS a9-64.akam.net.
157.128.in-addr.arpa. 5 NS a14-67.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0