Apex domains cannot have CNAME records.
Question and response
Question
dig getpostman.com. NS
Answer
getpostman.com. 5 NS ns-1526.awsdns-62.org.
getpostman.com. 5 NS ns-1696.awsdns-20.co.uk.
getpostman.com. 5 NS ns-37.awsdns-04.com.
getpostman.com. 5 NS ns-833.awsdns-40.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0