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