Apex domains cannot have CNAME records.
Question and response
Question
dig Sendgrid.net. NS
Answer
sendgrid.net. 2 NS ns13.dnsmadeeasy.com.
sendgrid.net. 2 NS ns11.dnsmadeeasy.com.
sendgrid.net. 2 NS ns10.dnsmadeeasy.com.
sendgrid.net. 2 NS ns12.dnsmadeeasy.com.
sendgrid.net. 2 NS ns15.dnsmadeeasy.com.
sendgrid.net. 2 NS ns14.dnsmadeeasy.com.
Authority
The response has no authority records.
Additional
ns10.dnsmadeeasy.com. 1 A 208.94.148.4
ns11.dnsmadeeasy.com. 1 A 208.80.124.4
ns12.dnsmadeeasy.com. 1 A 208.80.126.4
ns13.dnsmadeeasy.com. 1 A 208.80.125.4
ns14.dnsmadeeasy.com. 1 A 208.80.127.4
ns15.dnsmadeeasy.com. 1 A 208.94.149.4
ns10.dnsmadeeasy.com. 1 AAAA 2600:1800:10:0:0:0:0:1
ns11.dnsmadeeasy.com. 1 AAAA 2600:1801:11:0:0:0:0:1
ns12.dnsmadeeasy.com. 1 AAAA 2600:1802:12:0:0:0:0:1
ns13.dnsmadeeasy.com. 1 AAAA 2600:1801:13:0:0:0:0:1
ns14.dnsmadeeasy.com. 1 AAAA 2600:1802:14:0:0:0:0:1
ns15.dnsmadeeasy.com. 1 AAAA 2600:1800:15:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0