Apex domains cannot have CNAME records.
Question and response
Question
dig _spf.sparkpostmail.com. NS
Answer
_spf.sparkpostmail.com. 5 NS ns-1691.awsdns-19.co.uk.
_spf.sparkpostmail.com. 5 NS ns-628.awsdns-14.net.
_spf.sparkpostmail.com. 5 NS ns-269.awsdns-33.com.
_spf.sparkpostmail.com. 5 NS ns-1381.awsdns-44.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0