Apex domains cannot have CNAME records.
Question and response
Question
dig 100.pt. NS
Answer
100.pt. 5 NS niobe.100.pt.
100.pt. 5 NS zee.tuxdns.net.
100.pt. 5 NS trinity.tuxdns.com.
Authority
The response has no authority records.
Additional
zee.tuxdns.net. 5 A 193.163.151.100
niobe.100.pt. 5 A 185.83.251.132
trinity.tuxdns.com. 5 A 195.200.253.233
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0