Apex domains cannot have CNAME records.
Question and response
Question
dig cpu.lublin.pl. NS
Answer
cpu.lublin.pl. 5 NS ns4.lubman.pl.
cpu.lublin.pl. 5 NS ns3.lubman.pl.
cpu.lublin.pl. 5 NS ns1.lublin.pl.
cpu.lublin.pl. 5 NS ns2.lublin.pl.
Authority
The response has no authority records.
Additional
ns1.lublin.pl. 5 A 212.182.63.66
ns2.lublin.pl. 5 A 212.182.125.70
ns3.lubman.pl. 5 A 62.233.166.220
ns4.lubman.pl. 5 A 62.233.166.219
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0