Apex domains cannot have CNAME records.
Question and response
Question
dig improvmx.com. NS
Answer
improvmx.com. 4 NS logan.ns.cloudflare.com.
improvmx.com. 4 NS stevie.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0