Apex domains cannot have CNAME records.
Question and response
Question
dig 53.in-addr.arpa. NS
Answer
53.in-addr.arpa. 5 NS ns2.corpinter.de.
53.in-addr.arpa. 5 NS ns1.corpinter.net.
53.in-addr.arpa. 5 NS ns4.corpinter.de.
53.in-addr.arpa. 5 NS ns3.corpinter.net.
Authority
The response has no authority records.
Additional
ns1.corpinter.net. 5 A 141.113.0.53
ns2.corpinter.de. 5 A 141.113.8.53
ns3.corpinter.net. 5 A 141.113.136.53
ns4.corpinter.de. 5 A 141.113.168.53
ns1.corpinter.net. 5 AAAA 2a03:9e42:e000:1:0:0:0:53
ns2.corpinter.de. 5 AAAA 2a03:9e42:e010:1:0:0:0:53
ns4.corpinter.de. 5 AAAA 2a03:9e41:e101:1001:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0