Apex domains cannot have CNAME records.
Question and response
Question
dig tpnet.pl. NS
Answer
tpnet.pl. 5 NS ns2.corpnet.pl.
tpnet.pl. 5 NS ns3.corpnet.pl.
tpnet.pl. 5 NS szeloba.nask.waw.pl.
tpnet.pl. 5 NS dns2.man.lodz.pl.
tpnet.pl. 5 NS ns1.corpnet.pl.
tpnet.pl. 5 NS ns4.corpnet.pl.
Authority
The response has no authority records.
Additional
ns1.corpnet.pl. 4 A 80.50.50.100
ns2.corpnet.pl. 4 A 80.50.50.150
ns3.corpnet.pl. 4 A 80.50.50.200
ns4.corpnet.pl. 4 A 80.50.50.180
dns2.man.lodz.pl. 5 A 212.51.192.5
ns1.corpnet.pl. 4 AAAA 2a01:1700:2:1:0:0:0:3264
ns2.corpnet.pl. 4 AAAA 2a01:1700:3:2:0:0:0:3296
ns3.corpnet.pl. 4 AAAA 2a01:1700:2:3:0:0:0:32c8
ns4.corpnet.pl. 4 AAAA 2a01:1700:3:4:0:0:0:32b4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0