Apex domains cannot have CNAME records.
Question and response
Question
dig knack.com. NS
Answer
knack.com. 4 NS ns-1928.awsdns-49.co.uk.
knack.com. 4 NS ns-104.awsdns-13.com.
knack.com. 4 NS ns-716.awsdns-25.net.
knack.com. 4 NS ns-1312.awsdns-36.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0