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