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