Apex domains cannot have CNAME records.
Question and response
Question
dig pro-linuxpl.com. NS
Answer
pro-linuxpl.com. 5 NS ns3.cyberfolks.pl.
pro-linuxpl.com. 5 NS ns1.cyberfolks.pl.
pro-linuxpl.com. 5 NS ns2.cyberfolks.pl.
Authority
The response has no authority records.
Additional
ns2.cyberfolks.pl. 1 A 94.152.255.111
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0