Apex domains cannot have CNAME records.
Question and response
Question
dig sgh.waw.pl. NS
Answer
sgh.waw.pl. 5 NS eomer.nask.waw.pl.
sgh.waw.pl. 5 NS ns2.sgh.waw.pl.
sgh.waw.pl. 5 NS ns.icm.edu.pl.
sgh.waw.pl. 5 NS hermes.sgh.waw.pl.
Authority
The response has no authority records.
Additional
ns.icm.edu.pl. 5 A 212.87.14.39
ns2.sgh.waw.pl. 5 A 194.145.96.110
hermes.sgh.waw.pl. 5 A 194.145.96.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0