Apex domains cannot have CNAME records.
Question and response
Question
dig cerb.email. NS
Answer
cerb.email. 4 NS ns-1093.awsdns-08.org.
cerb.email. 4 NS ns-346.awsdns-43.com.
cerb.email. 4 NS ns-776.awsdns-33.net.
cerb.email. 4 NS ns-1628.awsdns-11.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0