Apex domains cannot have CNAME records.
Question and response
Question
dig ptc.com.pl. NS
Answer
ptc.com.pl. 5 NS ns4.ptc.com.pl.
ptc.com.pl. 5 NS ns5.ptc.com.pl.
ptc.com.pl. 5 NS ns3.ptc.com.pl.
Authority
The response has no authority records.
Additional
ns3.ptc.com.pl. 5 A 213.158.223.73
ns4.ptc.com.pl. 5 A 213.158.215.65
ns5.ptc.com.pl. 5 A 213.158.223.65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0