Apex domains cannot have CNAME records.
Question and response
Question
dig uni2.es. NS
Answer
uni2.es. 5 NS dns2.comtenidos.com.
uni2.es. 5 NS dns.comtenidos.com.
Authority
The response has no authority records.
Additional
dns.comtenidos.com. 5 A 85.62.229.140
dns2.comtenidos.com. 5 A 85.62.233.140
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0