Apex domains cannot have CNAME records.
Question and response
Question
dig servercore.mx. NS
Answer
servercore.mx. 5 NS c.ns.srv.cr.
servercore.mx. 5 NS d.ns.srv.cr.
servercore.mx. 5 NS a.ns.srv.cr.
servercore.mx. 5 NS b.ns.srv.cr.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0