Apex domains cannot have CNAME records.
Question and response
Question
dig calserver.eu. NS
Answer
calserver.eu. 3 NS ns1.cal.pl.
calserver.eu. 3 NS ns2.cal.pl.
Authority
The response has no authority records.
Additional
ns1.cal.pl. 3 A 45.138.106.10
ns2.cal.pl. 3 A 185.8.238.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0