Apex domains cannot have CNAME records.
Question and response
Question
dig 224-255.76.18.185.in-addr.arpa. NS
Answer
224-255.76.18.185.in-addr.arpa. 5 NS pdnsadmin.ficolo.net.
224-255.76.18.185.in-addr.arpa. 5 NS dns1.ficolo.net.
224-255.76.18.185.in-addr.arpa. 5 NS dns2.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
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0