Apex domains cannot have CNAME records.
Question and response
Question
dig sl.pt. NS
Answer
sl.pt. 5 NS dns02.sapo.pt.
sl.pt. 5 NS dns01.sapo.pt.
sl.pt. 5 NS ns.sapo.pt.
sl.pt. 5 NS ns2.sapo.pt.
Authority
The response has no authority records.
Additional
ns.sapo.pt. 5 A 212.55.154.202
ns2.sapo.pt. 5 A 212.55.154.194
dns01.sapo.pt. 5 A 213.13.28.116
dns02.sapo.pt. 5 A 213.13.30.116
dns01.sapo.pt. 5 AAAA 2001:8a0:2106:4:213:13:28:116
dns02.sapo.pt. 5 AAAA 2001:8a0:2206:4:213:13:30:116
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0