Apex domains cannot have CNAME records.
Question and response
Question
dig cerb.cloud. NS
Answer
cerb.cloud. 5 NS ns-2016.awsdns-60.co.uk.
cerb.cloud. 5 NS ns-813.awsdns-37.net.
cerb.cloud. 5 NS ns-421.awsdns-52.com.
cerb.cloud. 5 NS ns-1367.awsdns-42.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0