Apex domains cannot have CNAME records.
Question and response
Question
dig serv-net.pl. NS
Answer
serv-net.pl. 4 NS ns.vectra.pl.
serv-net.pl. 4 NS ns.multimedia.pl.
serv-net.pl. 4 NS ns.vectranet.pl.
Authority
The response has no authority records.
Additional
ns.vectra.pl. 4 A 78.88.74.19
ns.vectranet.pl. 4 A 78.88.74.20
ns.multimedia.pl. 4 A 78.88.74.21
ns.vectra.pl. 4 AAAA 2a00:9f00:1d01:f5:0:0:0:20
ns.vectranet.pl. 4 AAAA 2a00:9f00:1d01:f5:0:0:0:21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0