Apex domains cannot have CNAME records.
Question and response
Question
dig 125.2.81.in-addr.arpa. NS
Answer
125.2.81.in-addr.arpa. 5 NS auth.primary-dns.co.uk.
125.2.81.in-addr.arpa. 5 NS auth.secondary-dns.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0