Apex domains cannot have CNAME records.
Question and response
Question
dig 4.213.in-addr.arpa. NS
Answer
4.213.in-addr.arpa. 4 NS minerva.ttd.net.
4.213.in-addr.arpa. 4 NS artemis.ttd.net.
4.213.in-addr.arpa. 4 NS ns.ripe.net.
Authority
The response has no authority records.
Additional
artemis.ttd.net. 4 A 213.0.184.69
minerva.ttd.net. 4 A 213.0.184.68
artemis.ttd.net. 4 AAAA 2a02:9000:0:0:0:0:1:3
minerva.ttd.net. 4 AAAA 2a02:9000:0:0:0:0:1:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0