Apex domains cannot have CNAME records.
Question and response
Question
dig arpa.cam.ac.uk. NS
Answer
arpa.cam.ac.uk. 1 NS auth0.dns.cam.ac.uk.
arpa.cam.ac.uk. 1 NS auth1.dns.cam.ac.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0