Apex domains cannot have CNAME records.
Question and response
Question
dig zendesk.com. NS
Answer
zendesk.com. 5 NS ns-1213.awsdns-23.org.
zendesk.com. 5 NS ns-1858.awsdns-40.co.uk.
zendesk.com. 5 NS ns-444.awsdns-55.com.
zendesk.com. 5 NS ns-754.awsdns-30.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0