Apex domains cannot have CNAME records.
Question and response
Question
dig datium.es. NS
Answer
datium.es. 5 NS ns1027.ui-dns.de.
datium.es. 5 NS ns1030.ui-dns.com.
datium.es. 5 NS ns1055.ui-dns.org.
datium.es. 5 NS ns1089.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0