Apex domains cannot have CNAME records.
Question and response
Question
dig mkto-b0381.com. NS
Answer
mkto-b0381.com. 4 NS ns-1385.awsdns-45.org.
mkto-b0381.com. 4 NS ns-436.awsdns-54.com.
mkto-b0381.com. 4 NS ns-1071.awsdns-05.org.
mkto-b0381.com. 4 NS ns-1424.awsdns-50.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0