Apex domains cannot have CNAME records.
Question and response
Question
dig 1-255.238.91.212.in-addr.arpa. NS
Answer
1-255.238.91.212.in-addr.arpa. 5 NS ns7.dns-net.de.
1-255.238.91.212.in-addr.arpa. 5 NS ns0.dns-net.de.
Authority
The response has no authority records.
Additional
ns0.dns-net.de. 5 A 212.91.230.34
ns7.dns-net.de. 5 A 212.91.231.34
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0