Apex domains cannot have CNAME records.
Question and response
Question
dig mailers.a-lehdet.fi. NS
Answer
mailers.a-lehdet.fi. 5 NS ns2.viidakko.fi.
mailers.a-lehdet.fi. 5 NS ns1.viidakko.fi.
mailers.a-lehdet.fi. 5 NS ns4.viidakko.net.
mailers.a-lehdet.fi. 5 NS ns3.viidakko.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0