Apex domains cannot have CNAME records.
Question and response
Question
dig sendgrid.net. NS
Answer
sendgrid.net. 5 NS ns10.dnsmadeeasy.com.
sendgrid.net. 5 NS ns11.dnsmadeeasy.com.
sendgrid.net. 5 NS ns12.dnsmadeeasy.com.
sendgrid.net. 5 NS ns13.dnsmadeeasy.com.
sendgrid.net. 5 NS ns14.dnsmadeeasy.com.
sendgrid.net. 5 NS ns15.dnsmadeeasy.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0