Apex domains cannot have CNAME records.
Question and response
Question
dig mta.smrk.io. NS
Answer
mta.smrk.io. 5 NS ns-1865.awsdns-41.co.uk.
mta.smrk.io. 5 NS ns-1324.awsdns-37.org.
mta.smrk.io. 5 NS ns-77.awsdns-09.com.
mta.smrk.io. 5 NS ns-541.awsdns-03.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0