Apex domains cannot have CNAME records.
Question and response
Question
dig zadns.org. NS
Answer
zadns.org. 5 NS ns3.mydnscloud.com.
zadns.org. 5 NS ns4.mydnscloud.com.
zadns.org. 5 NS ns1.mydnscloud.com.
zadns.org. 5 NS ns2.mydnscloud.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0