Apex domains cannot have CNAME records.
Question and response
Question
dig static.cpe.netcabo.pt. NS
Answer
static.cpe.netcabo.pt. 5 NS ns1.zon.pt.
static.cpe.netcabo.pt. 5 NS ns2.zon.pt.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0