Apex domains cannot have CNAME records.
Question and response
Question
dig kb4.io. NS
Answer
kb4.io. 5 NS ns-337.awsdns-42.com.
kb4.io. 5 NS ns-2035.awsdns-62.co.uk.
kb4.io. 5 NS ns-1422.awsdns-49.org.
kb4.io. 5 NS ns-829.awsdns-39.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0