Apex domains cannot have CNAME records.
Question and response
Question
dig aena.es. NS
Answer
aena.es. 5 NS artemis.ttd.net.
aena.es. 5 NS dns12.aena.es.
aena.es. 5 NS dns6.aena.es.
aena.es. 5 NS dns7.aena.es.
aena.es. 5 NS dns13.aena.es.
Authority
The response has no authority records.
Additional
dns6.aena.es. 5 A 194.224.177.231
dns7.aena.es. 5 A 194.224.177.232
dns12.aena.es. 5 A 217.130.124.231
dns13.aena.es. 5 A 217.130.124.232
artemis.ttd.net. 5 A 213.0.184.69
artemis.ttd.net. 5 AAAA 2a02:9000:0:0:0:0:1:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0