Apex domains cannot have CNAME records.
Question and response
Question
dig man.poznan.pl. NS
Answer
man.poznan.pl. 5 NS rose.man.poznan.pl.
man.poznan.pl. 5 NS berberis.man.poznan.pl.
man.poznan.pl. 5 NS sunflower.man.poznan.pl.
Authority
The response has no authority records.
Additional
rose.man.poznan.pl. 5 A 150.254.173.3
berberis.man.poznan.pl. 5 A 212.126.0.1
sunflower.man.poznan.pl. 5 A 150.254.173.2
berberis.man.poznan.pl. 5 AAAA 2001:809:0:0:0:0:0:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0