Apex domains cannot have CNAME records.
Question and response
Question
dig flyio.net. NS
Answer
flyio.net. 5 NS ns-1111.awsdns-10.org.
flyio.net. 5 NS ns-1787.awsdns-31.co.uk.
flyio.net. 5 NS ns-506.awsdns-63.com.
flyio.net. 5 NS ns-757.awsdns-30.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0