Apex domains cannot have CNAME records.
Question and response
Question
dig 0-255.202.231.5.in-addr.arpa. NS
Answer
0-255.202.231.5.in-addr.arpa. 5 NS ns3.ghostgate.de.
0-255.202.231.5.in-addr.arpa. 5 NS ns2.ghostgate.de.
0-255.202.231.5.in-addr.arpa. 5 NS ns1.ghostgate.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0