Apex domains cannot have CNAME records.
Question and response
Question
dig slack.com. NS
Answer
slack.com. 5 NS ns-1901.awsdns-45.co.uk.
slack.com. 5 NS ns-1493.awsdns-58.org.
slack.com. 5 NS ns-166.awsdns-20.com.
slack.com. 5 NS ns-606.awsdns-11.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0