Apex domains cannot have CNAME records.
Question and response
Question
dig healthgrades.com. NS
Answer
healthgrades.com. 5 NS ns-1962.awsdns-53.co.uk.
healthgrades.com. 5 NS ns-202.awsdns-25.com.
healthgrades.com. 5 NS ns-846.awsdns-41.net.
healthgrades.com. 5 NS ns-1102.awsdns-09.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0