Apex domains cannot have CNAME records.
Question and response
Question
dig korpdns.net. NS
Answer
korpdns.net. 5 NS istanbul.ns.korpcloud.com.
korpdns.net. 5 NS falkenstein.ns.korpcloud.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0