Apex domains cannot have CNAME records.
Question and response
Question
dig s1.zenbox.pl. NS
Answer
s1.zenbox.pl. 5 NS ns1.zenbox.pl.
s1.zenbox.pl. 5 NS ns2.zenbox.pl.
Authority
The response has no authority records.
Additional
ns1.zenbox.pl. 5 A 94.152.254.108
ns2.zenbox.pl. 5 A 94.152.255.108
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0