Apex domains cannot have CNAME records.
Question and response
Question
dig sendpulse.com. NS
Answer
sendpulse.com. 5 NS ns-708.awsdns-24.net.
sendpulse.com. 5 NS ns-1993.awsdns-57.co.uk.
sendpulse.com. 5 NS ns-181.awsdns-22.com.
sendpulse.com. 5 NS ns-1181.awsdns-19.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0