Apex domains cannot have CNAME records.
Question and response
Question
dig knowbe4.com. NS
Answer
knowbe4.com. 5 NS ns-853.awsdns-42.net.
knowbe4.com. 5 NS ns-38.awsdns-04.com.
knowbe4.com. 5 NS ns-1882.awsdns-43.co.uk.
knowbe4.com. 5 NS ns-1036.awsdns-01.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0