Apex domains cannot have CNAME records.
Question and response
Question
dig threatsim.com. NS
Answer
threatsim.com. 4 NS ns-203.awsdns-25.com.
threatsim.com. 4 NS ns-876.awsdns-45.net.
threatsim.com. 4 NS ns-1410.awsdns-48.org.
threatsim.com. 4 NS ns-1653.awsdns-14.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0