Apex domains cannot have CNAME records.
Question and response
Question
dig mainnameserver.com. NS
Answer
mainnameserver.com. 1 NS ns-212-a.gandi.net.
mainnameserver.com. 1 NS ns-172-b.gandi.net.
mainnameserver.com. 1 NS ns-74-c.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0