Apex domains cannot have CNAME records.
Question and response
Question
dig core.icm.edu.pl. NS
Answer
core.icm.edu.pl. 5 NS ns1.net.icm.edu.pl.
core.icm.edu.pl. 5 NS ns2.net.icm.edu.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0