Apex domains cannot have CNAME records.
Question and response
Question
dig nubedns.com. NS
Answer
nubedns.com. 4 NS ns3.nubedns.com.
nubedns.com. 4 NS ns1.nubedns.com.
nubedns.com. 4 NS ns2.nubedns.com.
nubedns.com. 4 NS ns4.nubedns.com.
Authority
The response has no authority records.
Additional
ns1.nubedns.com. 4 A 185.123.204.25
ns2.nubedns.com. 4 A 31.207.89.25
ns3.nubedns.com. 4 A 5.135.162.106
ns4.nubedns.com. 4 A 107.191.39.243
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0