Apex domains cannot have CNAME records.
Question and response
Question
dig v6.ficolo.net. NS
Answer
v6.ficolo.net. 5 NS dns2.ficolo.net.
v6.ficolo.net. 5 NS dns1.ficolo.net.
v6.ficolo.net. 5 NS dns3.ficolo.net.
Authority
The response has no authority records.
Additional
dns1.ficolo.net. 5 A 164.215.32.84
dns2.ficolo.net. 5 A 164.215.32.222
dns3.ficolo.net. 5 A 185.20.137.65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0