Apex domains cannot have CNAME records.
Question and response
Question
dig upc.es. NS
Answer
upc.es. 5 NS sun.rediris.es.
upc.es. 5 NS ns2.upc.edu.
upc.es. 5 NS ns1.upc.edu.
upc.es. 5 NS chico.rediris.es.
Authority
The response has no authority records.
Additional
ns1.upc.edu. 5 A 147.83.0.1
ns2.upc.edu. 5 A 147.83.0.2
sun.rediris.es. 5 A 199.184.182.1
chico.rediris.es. 5 A 162.219.54.2
sun.rediris.es. 5 AAAA 2620:171:808:0:0:0:0:1
chico.rediris.es. 5 AAAA 2620:10a:80eb:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0