Apex domains cannot have CNAME records.
Question and response
Question
dig ent-cac1-trusted.docebomail.com. NS
Answer
ent-cac1-trusted.docebomail.com. 5 NS ns-1878.awsdns-42.co.uk.
ent-cac1-trusted.docebomail.com. 5 NS ns-573.awsdns-07.net.
ent-cac1-trusted.docebomail.com. 5 NS ns-258.awsdns-32.com.
ent-cac1-trusted.docebomail.com. 5 NS ns-1092.awsdns-08.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0