Apex domains cannot have CNAME records.
Question and response
Question
dig mxa.freshdesk.com. NS
Answer
mxa.freshdesk.com. 5 NS ns-996.awsdns-60.net.
mxa.freshdesk.com. 5 NS ns-1956.awsdns-52.co.uk.
mxa.freshdesk.com. 5 NS ns-369.awsdns-46.com.
mxa.freshdesk.com. 5 NS ns-1144.awsdns-15.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0