Apex domains cannot have CNAME records.
Question and response
Question
dig pwr.edu.pl. NS
Answer
pwr.edu.pl. 5 NS ns2.net.icm.edu.pl.
pwr.edu.pl. 5 NS dns1.pwr.edu.pl.
pwr.edu.pl. 5 NS ns1.net.icm.edu.pl.
pwr.edu.pl. 5 NS dns.pwr.edu.pl.
pwr.edu.pl. 5 NS sun2.pwr.wroc.pl.
pwr.edu.pl. 5 NS dns4.pwr.edu.pl.
pwr.edu.pl. 5 NS dns3.pwr.edu.pl.
Authority
The response has no authority records.
Additional
dns.pwr.edu.pl. 5 A 156.17.18.10
dns1.pwr.edu.pl. 5 A 156.17.140.3
dns3.pwr.edu.pl. 5 A 156.17.197.98
dns4.pwr.edu.pl. 5 A 156.17.140.4
sun2.pwr.wroc.pl. 5 A 156.17.5.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0