Apex domains cannot have CNAME records.
Question and response
Question
dig ent-aps1.docebomail.com. NS
Answer
ent-aps1.docebomail.com. 5 NS ns-1727.awsdns-23.co.uk.
ent-aps1.docebomail.com. 5 NS ns-728.awsdns-27.net.
ent-aps1.docebomail.com. 5 NS ns-33.awsdns-04.com.
ent-aps1.docebomail.com. 5 NS ns-1521.awsdns-62.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0