Apex domains cannot have CNAME records.
Question and response
Question
dig optimalblue.com. NS
Answer
optimalblue.com. 4 NS ns-1854.awsdns-39.co.uk.
optimalblue.com. 4 NS ns-532.awsdns-02.net.
optimalblue.com. 4 NS ns-79.awsdns-09.com.
optimalblue.com. 4 NS ns-1288.awsdns-33.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0