Apex domains cannot have CNAME records.
Question and response
Question
dig grin.hu. NS
Answer
grin.hu. 5 NS ns2.grin.hu.
grin.hu. 5 NS ns0.grin.hu.
grin.hu. 5 NS ns.tolna.net.
grin.hu. 5 NS ns.is.hu.
grin.hu. 5 NS ns1.grin.hu.
Authority
The response has no authority records.
Additional
ns.tolna.net. 5 A 91.146.167.221
ns0.grin.hu. 5 A 91.146.180.88
ns1.grin.hu. 5 A 91.146.167.221
ns2.grin.hu. 5 A 91.146.180.88
ns.tolna.net. 5 AAAA 2a01:5d0:0:2:0:0:0:3
ns0.grin.hu. 5 AAAA 2a01:5d0:1:10:0:0:0:8053
ns1.grin.hu. 5 AAAA 2a01:5d0:0:2:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0