Apex domains cannot have CNAME records.
Question and response
Question
dig saas.atlassian.com. NS
Answer
saas.atlassian.com. 5 NS ns-1208.awsdns-23.org.
saas.atlassian.com. 5 NS ns-711.awsdns-24.net.
saas.atlassian.com. 5 NS ns-187.awsdns-23.com.
saas.atlassian.com. 5 NS ns-1551.awsdns-01.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0