Apex domains cannot have CNAME records.
Question and response
Question
dig sigstr.com. NS
Answer
sigstr.com. 5 NS ns-809.awsdns-37.net.
sigstr.com. 5 NS ns-412.awsdns-51.com.
sigstr.com. 5 NS ns-1703.awsdns-20.co.uk.
sigstr.com. 5 NS ns-1063.awsdns-04.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0