Apex domains cannot have CNAME records.
Question and response
Question
dig voltera.fi. NS
Answer
voltera.fi. 5 NS ns.nesteoil.com.
voltera.fi. 5 NS ns2.nesteoil.com.
voltera.fi. 5 NS ns1.nesteoil.com.
Authority
The response has no authority records.
Additional
ns.nesteoil.com. 5 A 132.155.58.8
ns1.nesteoil.com. 5 A 132.155.58.9
ns2.nesteoil.com. 5 A 132.155.58.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0