Apex domains cannot have CNAME records.
Question and response
Question
dig sparkpostmail.com. NS
Answer
sparkpostmail.com. 5 NS ns-1361.awsdns-42.org.
sparkpostmail.com. 5 NS ns-1855.awsdns-39.co.uk.
sparkpostmail.com. 5 NS ns-304.awsdns-38.com.
sparkpostmail.com. 5 NS ns-775.awsdns-32.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0