Apex domains cannot have CNAME records.
Question and response
Question
dig upcloud.com. NS
Answer
upcloud.com. 5 NS dns1.upcloud.com.
upcloud.com. 5 NS dns3.upcloud.com.
upcloud.com. 5 NS dns2.upcloud.com.
Authority
The response has no authority records.
Additional
dns1.upcloud.com. 5 A 162.159.8.139
dns2.upcloud.com. 5 A 162.159.9.99
dns3.upcloud.com. 5 A 162.159.10.192
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0