Apex domains cannot have CNAME records.
Question and response
Question
dig use1-ecs-trusted.docebomail.com. NS
Answer
use1-ecs-trusted.docebomail.com. 5 NS ns-340.awsdns-42.com.
use1-ecs-trusted.docebomail.com. 5 NS ns-1574.awsdns-04.co.uk.
use1-ecs-trusted.docebomail.com. 5 NS ns-1521.awsdns-62.org.
use1-ecs-trusted.docebomail.com. 5 NS ns-776.awsdns-33.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0