Apex domains cannot have CNAME records.
Question and response
Question
dig klaviyo.com. NS
Answer
klaviyo.com. 5 NS ns-963.awsdns-56.net.
klaviyo.com. 5 NS ns-1555.awsdns-02.co.uk.
klaviyo.com. 5 NS ns-224.awsdns-28.com.
klaviyo.com. 5 NS ns-1372.awsdns-43.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0