Apex domains cannot have CNAME records.
Question and response
Question
dig mxau.freshdesk.com. NS
Answer
mxau.freshdesk.com. 5 NS ns-1224.awsdns-25.org.
mxau.freshdesk.com. 5 NS ns-1866.awsdns-41.co.uk.
mxau.freshdesk.com. 5 NS ns-144.awsdns-18.com.
mxau.freshdesk.com. 5 NS ns-543.awsdns-03.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0