Apex domains cannot have CNAME records.
Question and response
Question
dig 90.216.91.in-addr.arpa. NS
Answer
90.216.91.in-addr.arpa. 5 NS ns1.iphh.de.
90.216.91.in-addr.arpa. 5 NS ns2.iphh.de.
Authority
The response has no authority records.
Additional
ns1.iphh.de. 5 A 213.128.128.33
ns2.iphh.de. 5 A 213.128.129.66
ns1.iphh.de. 5 AAAA 2001:868:0:180:0:0:1:1
ns2.iphh.de. 5 AAAA 2001:868:0:181:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0