Apex domains cannot have CNAME records.
Question and response
Question
dig websitetoolbox.com. NS
Answer
websitetoolbox.com. 2 NS ns-737.awsdns-28.net.
websitetoolbox.com. 2 NS ns-1509.awsdns-60.org.
websitetoolbox.com. 2 NS ns-1806.awsdns-33.co.uk.
websitetoolbox.com. 2 NS ns-52.awsdns-06.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0