Apex domains cannot have CNAME records.
Question and response
Question
dig siel.ninja. NS
Answer
siel.ninja. 5 NS dns3.siel.si.
siel.ninja. 5 NS dns1.siel.si.
siel.ninja. 5 NS dns2.siel.si.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0