Apex domains cannot have CNAME records.
Question and response
Question
dig 8-15.12.155.90.in-addr.arpa. NS
Answer
8-15.12.155.90.in-addr.arpa. 5 NS ns-global.kjsl.com.
8-15.12.155.90.in-addr.arpa. 5 NS ns2.terryburton.co.uk.
8-15.12.155.90.in-addr.arpa. 5 NS ns1.terryburton.co.uk.
8-15.12.155.90.in-addr.arpa. 5 NS ns2.afraid.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0