Apex domains cannot have CNAME records.
Question and response
Question
dig heldenvannu.net. NS
Answer
heldenvannu.net. 5 NS ns-1325.awsdns-37.org.
heldenvannu.net. 5 NS ns-693.awsdns-22.net.
heldenvannu.net. 5 NS ns-382.awsdns-47.com.
heldenvannu.net. 5 NS ns-1847.awsdns-38.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0