Apex domains cannot have CNAME records.
Question and response
Question
dig sieci.net.pl. NS
Answer
sieci.net.pl. 5 NS dns.eldip.pl.
sieci.net.pl. 5 NS dns2.obslugasieci.pl.
sieci.net.pl. 5 NS dns1.obslugasieci.pl.
Authority
The response has no authority records.
Additional
dns.eldip.pl. 5 A 31.182.7.101
dns1.obslugasieci.pl. 5 A 83.12.129.250
dns2.obslugasieci.pl. 5 A 83.12.129.251
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0