Apex domains cannot have CNAME records.
Question and response
Question
dig sucuridns.com. NS
Answer
sucuridns.com. 5 NS ns-694.awsdns-22.net.
sucuridns.com. 5 NS ns-1923.awsdns-48.co.uk.
sucuridns.com. 5 NS ns-82.awsdns-10.com.
sucuridns.com. 5 NS ns-1338.awsdns-39.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0