Apex domains cannot have CNAME records.
Question and response
Question
dig wombatsecurity.com. NS
Answer
wombatsecurity.com. 5 NS ns-1119.awsdns-11.org.
wombatsecurity.com. 5 NS ns-423.awsdns-52.com.
wombatsecurity.com. 5 NS ns-734.awsdns-27.net.
wombatsecurity.com. 5 NS ns-1918.awsdns-47.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0