Apex domains cannot have CNAME records.
Question and response
Question
dig in-addr.arpa. NS
Answer
in-addr.arpa. 5 NS b.in-addr-servers.arpa.
in-addr.arpa. 5 NS e.in-addr-servers.arpa.
in-addr.arpa. 5 NS d.in-addr-servers.arpa.
in-addr.arpa. 5 NS a.in-addr-servers.arpa.
in-addr.arpa. 5 NS f.in-addr-servers.arpa.
in-addr.arpa. 5 NS c.in-addr-servers.arpa.
Authority
The response has no authority records.
Additional
a.in-addr-servers.arpa. 100040 A 199.180.182.53
b.in-addr-servers.arpa. 119794 A 199.253.183.183
c.in-addr-servers.arpa. 21479 A 196.216.169.10
d.in-addr-servers.arpa. 2626 A 200.10.60.53
e.in-addr-servers.arpa. 143911 A 203.119.86.101
f.in-addr-servers.arpa. 163980 A 193.0.9.1
a.in-addr-servers.arpa. 100040 AAAA 2620:37:e000:0:0:0:0:53
b.in-addr-servers.arpa. 119794 AAAA 2001:500:87:0:0:0:0:87
c.in-addr-servers.arpa. 21479 AAAA 2001:43f8:110:0:0:0:0:10
d.in-addr-servers.arpa. 2626 AAAA 2001:13c7:7010:0:0:0:0:53
e.in-addr-servers.arpa. 143911 AAAA 2001:dd8:6:0:0:0:0:101
f.in-addr-servers.arpa. 163980 AAAA 2a13:27c0:30:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0