Apex domains cannot have CNAME records.
Question and response
Question
dig bg.szczecin.pl. NS
Answer
bg.szczecin.pl. 5 NS ns1.twisted4life.com.
bg.szczecin.pl. 5 NS fns1.42.pl.
bg.szczecin.pl. 5 NS main.bibl.univ.szczecin.pl.
bg.szczecin.pl. 5 NS bg.bibl.univ.szczecin.pl.
bg.szczecin.pl. 5 NS fns2.42.pl.
Authority
The response has no authority records.
Additional
ns1.twisted4life.com. 5 A 103.13.122.142
fns1.42.pl. 5 A 79.98.145.34
fns2.42.pl. 5 A 51.38.99.90
fns2.42.pl. 5 AAAA 2001:41d0:701:1100:0:0:0:66c
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0