Apex domains cannot have CNAME records.
Question and response
Question
dig sucuri.net. NS
Answer
sucuri.net. 4 NS 15.sucuridns.com.
sucuri.net. 4 NS 11.sucuridns.com.
sucuri.net. 4 NS 10.sucuridns.com.
sucuri.net. 4 NS 12.sucuridns.com.
sucuri.net. 4 NS 13.sucuridns.com.
sucuri.net. 4 NS 14.sucuridns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0