Apex domains cannot have CNAME records.
Question and response
Question
dig flight.sparkpost.com. NS
Answer
flight.sparkpost.com. 4 NS ns-422.awsdns-52.com.
flight.sparkpost.com. 4 NS ns-1788.awsdns-31.co.uk.
flight.sparkpost.com. 4 NS ns-1179.awsdns-19.org.
flight.sparkpost.com. 4 NS ns-774.awsdns-32.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0