Apex domains cannot have CNAME records.
Question and response
Question
dig ip.interplus.com.pl. NS
Answer
ip.interplus.com.pl. 5 NS dns2.interplus.com.pl.
ip.interplus.com.pl. 5 NS dns1.interplus.com.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0