Apex domains cannot have CNAME records.
Question and response
Question
dig bydgoszcz.sprint.pl. NS
Answer
bydgoszcz.sprint.pl. 5 NS netra.gdansk.sprint.pl.
bydgoszcz.sprint.pl. 5 NS ninthgate.bydgoszcz.sprint.pl.
Authority
The response has no authority records.
Additional
netra.gdansk.sprint.pl. 5 A 213.192.74.77
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0