Apex domains cannot have CNAME records.
Question and response
Question
dig supermedia.pl. NS
Answer
supermedia.pl. 5 NS dns2.supermedia.pl.
supermedia.pl. 5 NS dns1.supermedia.pl.
supermedia.pl. 5 NS dns3.supermedia.pl.
Authority
The response has no authority records.
Additional
dns1.supermedia.pl. 5 A 212.180.140.1
dns2.supermedia.pl. 5 A 85.232.245.83
dns3.supermedia.pl. 5 A 85.10.199.116
dns1.supermedia.pl. 5 AAAA 2001:4c30:100:1:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0