Apex domains cannot have CNAME records.
Question and response
Question
dig internet.jettel.pl. NS
Answer
internet.jettel.pl. 5 NS dns1.jettel.pl.
internet.jettel.pl. 5 NS dns3.jettel.pl.
internet.jettel.pl. 5 NS dns2.jettel.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0