Apex domains cannot have CNAME records.
Question and response
Question
dig onice.io. NS
Answer
onice.io. 4 NS dns1.ttc.cz.apptocloud.net.
onice.io. 4 NS dns2.ttc.cz.apptocloud.net.
onice.io. 4 NS dns1.h90.cz.apptocloud.net.
Authority
The response has no authority records.
Additional
dns1.h90.cz.apptocloud.net. 4 A 130.185.180.216
dns1.ttc.cz.apptocloud.net. 4 A 185.138.220.7
dns2.ttc.cz.apptocloud.net. 4 A 185.138.220.8
dns1.h90.cz.apptocloud.net. 4 AAAA 2a03:b280:0:0:216:3eff:fe00:bd8
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0