Apex domains cannot have CNAME records.
Question and response
Question
dig bt.es. NS
Answer
bt.es. 5 NS ns2.bticglobal.net.
bt.es. 5 NS ns1.bticglobal.net.
Authority
The response has no authority records.
Additional
ns1.bticglobal.net. 5 A 195.99.66.211
ns2.bticglobal.net. 5 A 195.99.65.212
ns1.bticglobal.net. 5 AAAA 2a00:2000:206f:5:0:0:0:14f
ns2.bticglobal.net. 5 AAAA 2a00:2000:2073:2:0:0:0:4f
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0