Apex domains cannot have CNAME records.
Question and response
Question
dig ad. NS
Answer
ad. 1 NS anycast23.irondns.net.
ad. 1 NS anycast9.irondns.net.
ad. 1 NS ad.ns.nic.es.
ad. 1 NS ns3.nic.fr.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0