Apex domains cannot have CNAME records.
Question and response
Question
dig formstack.com. NS
Answer
formstack.com. 5 NS ns-500.awsdns-62.com.
formstack.com. 5 NS ns-2016.awsdns-60.co.uk.
formstack.com. 5 NS ns-1400.awsdns-47.org.
formstack.com. 5 NS ns-701.awsdns-23.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0