Apex domains cannot have CNAME records.
Question and response
Question
dig 238.91.212.in-addr.arpa. NS
Answer
238.91.212.in-addr.arpa. 1 NS ns1.dns-net.de.
238.91.212.in-addr.arpa. 1 NS ns7.dns-net.de.
Authority
The response has no authority records.
Additional
ns7.dns-net.de. 1 A 212.91.231.34
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0