Apex domains cannot have CNAME records.
Question and response
Question
dig slaskdatacenter.pl. NS
Answer
slaskdatacenter.pl. 5 NS dns1.slaskdatacenter.pl.
slaskdatacenter.pl. 5 NS dns3.slaskdatacenter.pl.
slaskdatacenter.pl. 5 NS dns2.slaskdatacenter.pl.
Authority
The response has no authority records.
Additional
dns1.slaskdatacenter.pl. 5 A 178.19.104.18
dns2.slaskdatacenter.pl. 5 A 178.19.109.186
dns3.slaskdatacenter.pl. 5 A 178.19.110.186
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0