Apex domains cannot have CNAME records.
Question and response
Question
dig 12.155.90.in-addr.arpa. NS
Answer
12.155.90.in-addr.arpa. 3 NS auth.primary-dns.co.uk.
12.155.90.in-addr.arpa. 3 NS auth.secondary-dns.co.uk.
Authority
The response has no authority records.
Additional
auth.primary-dns.co.uk. 2 A 194.4.174.2
auth.secondary-dns.co.uk. 2 A 194.4.173.2
auth.primary-dns.co.uk. 2 AAAA 2001:8b0:0:30:0:0:51bb:1e2a
auth.secondary-dns.co.uk. 2 AAAA 2001:8b0:0:81:0:0:51bb:5121
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0