Apex domains cannot have CNAME records.
Question and response
Question
dig ar.pt. NS
Answer
ar.pt. 5 NS dnsh04.artelecom.pt.
ar.pt. 5 NS dnsh03.artelecom.pt.
ar.pt. 5 NS dnsh01.artelecom.pt.
ar.pt. 5 NS dnsh02.artelecom.pt.
Authority
The response has no authority records.
Additional
dnsh01.artelecom.pt. 5 A 213.141.4.251
dnsh02.artelecom.pt. 5 A 213.141.27.251
dnsh03.artelecom.pt. 5 A 213.141.4.252
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0