Apex domains cannot have CNAME records.
Question and response
Question
dig ambushadvertising.com. NS
Answer
ambushadvertising.com. 5 NS ns-158.awsdns-19.com.
ambushadvertising.com. 5 NS ns-697.awsdns-23.net.
ambushadvertising.com. 5 NS ns-506.awsdns-63.com.
ambushadvertising.com. 5 NS ns-1430.awsdns-50.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0