Apex domains cannot have CNAME records.
Question and response
Question
dig gct.descartes.com. NS
Answer
gct.descartes.com. 5 NS ns-1120.awsdns-12.org.
gct.descartes.com. 5 NS ns-813.awsdns-37.net.
gct.descartes.com. 5 NS ns-313.awsdns-39.com.
gct.descartes.com. 5 NS ns-1691.awsdns-19.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0