Apex domains cannot have CNAME records.
Question and response
Question
dig internetdsl.tpnet.pl. NS
Answer
internetdsl.tpnet.pl. 5 NS ns1.tpnet.pl.
internetdsl.tpnet.pl. 5 NS ns2.tpnet.pl.
internetdsl.tpnet.pl. 5 NS ns3.tpnet.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0