Apex domains cannot have CNAME records.
Question and response
Question
dig mgw01.reputation.onclusive.com. NS
Answer
mgw01.reputation.onclusive.com. 1 NS ns-1281.awsdns-32.org.
mgw01.reputation.onclusive.com. 1 NS ns-1925.awsdns-48.co.uk.
mgw01.reputation.onclusive.com. 1 NS ns-290.awsdns-36.com.
mgw01.reputation.onclusive.com. 1 NS ns-907.awsdns-49.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0