Apex domains cannot have CNAME records.
Question and response
Question
dig ip6-servers.arpa. NS
Answer
ip6-servers.arpa. 5 NS f.ip6-servers.arpa.
ip6-servers.arpa. 5 NS c.ip6-servers.arpa.
ip6-servers.arpa. 5 NS d.ip6-servers.arpa.
ip6-servers.arpa. 5 NS a.ip6-servers.arpa.
ip6-servers.arpa. 5 NS b.ip6-servers.arpa.
ip6-servers.arpa. 5 NS e.ip6-servers.arpa.
Authority
The response has no authority records.
Additional
a.ip6-servers.arpa. 117874 A 199.180.182.53
b.ip6-servers.arpa. 117874 A 199.253.182.182
c.ip6-servers.arpa. 172400 A 196.216.169.11
d.ip6-servers.arpa. 117874 A 200.7.86.53
e.ip6-servers.arpa. 117874 A 203.119.86.101
f.ip6-servers.arpa. 117874 A 193.0.9.2
a.ip6-servers.arpa. 117874 AAAA 2620:37:e000:0:0:0:0:53
b.ip6-servers.arpa. 117874 AAAA 2001:500:86:0:0:0:0:86
c.ip6-servers.arpa. 172400 AAAA 2001:43f8:110:0:0:0:0:11
d.ip6-servers.arpa. 117874 AAAA 2001:13c7:7012:0:0:0:0:53
e.ip6-servers.arpa. 117874 AAAA 2001:dd8:6:0:0:0:0:101
f.ip6-servers.arpa. 117874 AAAA 2a13:27c0:30:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0