Apex domains cannot have CNAME records.
Question and response
Question
dig ent-cac1.docebomail.com. NS
Answer
ent-cac1.docebomail.com. 5 NS ns-1418.awsdns-49.org.
ent-cac1.docebomail.com. 5 NS ns-1855.awsdns-39.co.uk.
ent-cac1.docebomail.com. 5 NS ns-491.awsdns-61.com.
ent-cac1.docebomail.com. 5 NS ns-1007.awsdns-61.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0