Apex domains cannot have CNAME records.
Question and response
Question
dig barcelonatech-upc.eu. NS
Answer
barcelonatech-upc.eu. 1 NS ns1.upc.edu.
barcelonatech-upc.eu. 1 NS ns2.upc.edu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0