Apex domains cannot have CNAME records.
Question and response
Question
dig altostrat.io. NS
Answer
altostrat.io. 5 NS ns-961.awsdns-56.net.
altostrat.io. 5 NS ns-210.awsdns-26.com.
altostrat.io. 5 NS ns-1795.awsdns-32.co.uk.
altostrat.io. 5 NS ns-1177.awsdns-19.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0