Apex domains cannot have CNAME records.
Question and response
Question
dig ent-apse2.docebomail.com. NS
Answer
ent-apse2.docebomail.com. 5 NS ns-1762.awsdns-28.co.uk.
ent-apse2.docebomail.com. 5 NS ns-1163.awsdns-17.org.
ent-apse2.docebomail.com. 5 NS ns-410.awsdns-51.com.
ent-apse2.docebomail.com. 5 NS ns-744.awsdns-29.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0