Apex domains cannot have CNAME records.
Question and response
Question
dig omdc.pl. NS
Answer
omdc.pl. 5 NS ns5.omdc.pl.
omdc.pl. 5 NS ns2.omdc.pl.
omdc.pl. 5 NS ns3.omdc.pl.
omdc.pl. 5 NS ns1.omdc.pl.
omdc.pl. 5 NS ns4.omdc.pl.
omdc.pl. 5 NS ns6.omdc.pl.
Authority
The response has no authority records.
Additional
ns1.omdc.pl. 5 A 92.222.117.117
ns2.omdc.pl. 5 A 92.222.227.227
ns3.omdc.pl. 5 A 146.59.30.177
ns4.omdc.pl. 5 A 146.59.88.211
ns5.omdc.pl. 5 A 192.95.0.222
ns6.omdc.pl. 5 A 198.50.174.202
ns1.omdc.pl. 5 AAAA 2001:41d0:a:5d69:1111:1111:1111:1111
ns2.omdc.pl. 5 AAAA 2001:41d0:203:7c03:2222:2222:2222:2222
ns3.omdc.pl. 5 AAAA 2001:41d0:602:302:3333:3333:3333:3333
ns4.omdc.pl. 5 AAAA 2001:41d0:602:2251:4444:4444:4444:4444
ns5.omdc.pl. 5 AAAA 2607:5300:60:1524:5555:5555:5555:5555
ns6.omdc.pl. 5 AAAA 2607:5300:60:1524:6666:6666:6666:6666
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0