Apex domains cannot have CNAME records.
Question and response
Question
dig pwr.wroc.pl. NS
Answer
pwr.wroc.pl. 5 NS wask.wask.wroc.pl.
pwr.wroc.pl. 5 NS ns2.net.icm.edu.pl.
pwr.wroc.pl. 5 NS dns.pwr.wroc.pl.
pwr.wroc.pl. 5 NS dns2.pwr.wroc.pl.
pwr.wroc.pl. 5 NS dns1.pwr.edu.pl.
pwr.wroc.pl. 5 NS ns1.net.icm.edu.pl.
pwr.wroc.pl. 5 NS dns1.pwr.wroc.pl.
pwr.wroc.pl. 5 NS sun2.pwr.wroc.pl.
pwr.wroc.pl. 5 NS dns3.pwr.edu.pl.
pwr.wroc.pl. 5 NS dns4.pwr.edu.pl.
Authority
The response has no authority records.
Additional
sun2.pwr.wroc.pl. 5 A 156.17.5.2
wask.wask.wroc.pl. 5 A 156.17.254.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0