Apex domains cannot have CNAME records.
Question and response
Question
dig gcore.es. NS
Answer
gcore.es. 5 NS poit15.ns3.nameshift.com.
gcore.es. 5 NS ns1.nameshift.com.
gcore.es. 5 NS ns2.nameshift.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0