Apex domains cannot have CNAME records.
Question and response
Question
dig promdns.net. NS
Answer
promdns.net. 5 NS ns1.promdns.net.
promdns.net. 5 NS ns3.promdns.net.
promdns.net. 5 NS ns2.promdns.net.
Authority
The response has no authority records.
Additional
ns1.promdns.net. 5 A 193.34.169.3
ns2.promdns.net. 5 A 193.242.216.178
ns3.promdns.net. 5 A 18.195.201.38
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0