Apex domains cannot have CNAME records.
Question and response
Question
dig 232.in-addr.arpa. NS
Answer
232.in-addr.arpa. 5 NS b.iana-servers.net.
232.in-addr.arpa. 5 NS a.iana-servers.net.
232.in-addr.arpa. 5 NS ns.icann.org.
232.in-addr.arpa. 5 NS c.iana-servers.net.
Authority
The response has no authority records.
Additional
a.iana-servers.net. 4 A 199.43.135.53
b.iana-servers.net. 4 A 199.43.133.53
c.iana-servers.net. 4 A 199.43.134.53
ns.icann.org. 4 A 199.4.138.53
a.iana-servers.net. 4 AAAA 2001:500:8f:0:0:0:0:53
b.iana-servers.net. 4 AAAA 2001:500:8d:0:0:0:0:53
c.iana-servers.net. 4 AAAA 2001:500:8e:0:0:0:0:53
ns.icann.org. 4 AAAA 2001:500:89:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0