Apex domains cannot have CNAME records.
Question and response
Question
dig atlassian.net. NS
Answer
atlassian.net. 5 NS ns-110.awsdns-13.com.
atlassian.net. 5 NS ns-573.awsdns-07.net.
atlassian.net. 5 NS ns-1339.awsdns-39.org.
atlassian.net. 5 NS ns-1977.awsdns-55.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0