Apex domains cannot have CNAME records.
Question and response
Question
dig openinvoice.com. NS
Answer
openinvoice.com. 5 NS ns-902.awsdns-48.net.
openinvoice.com. 5 NS ns-405.awsdns-50.com.
openinvoice.com. 5 NS ns-1856.awsdns-40.co.uk.
openinvoice.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