Apex domains cannot have CNAME records.
Question and response
Question
dig sproutsocial.com. NS
Answer
sproutsocial.com. 5 NS ns-1770.awsdns-29.co.uk.
sproutsocial.com. 5 NS ns-1475.awsdns-56.org.
sproutsocial.com. 5 NS ns-109.awsdns-13.com.
sproutsocial.com. 5 NS ns-851.awsdns-42.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0