Apex domains cannot have CNAME records.
Question and response
Question
dig 12.86.in-addr.arpa. NS
Answer
12.86.in-addr.arpa. 2 NS ns2.virginmedia.net.
12.86.in-addr.arpa. 2 NS ns3.virginmedia.net.
12.86.in-addr.arpa. 2 NS ns4.virginmedia.net.
12.86.in-addr.arpa. 2 NS ns1.virginmedia.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0