Apex domains cannot have CNAME records.
Question and response
Question
dig sparkpost.com. NS
Answer
sparkpost.com. 5 NS ns-470.awsdns-58.com.
sparkpost.com. 5 NS ns-1923.awsdns-48.co.uk.
sparkpost.com. 5 NS ns-1518.awsdns-61.org.
sparkpost.com. 5 NS ns-520.awsdns-01.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0