Apex domains cannot have CNAME records.
Question and response
Question
dig 7.100.145.in-addr.arpa. NS
Answer
7.100.145.in-addr.arpa. 5 NS nsauth2.surfsara.nl.
7.100.145.in-addr.arpa. 5 NS nsauth1.surfsara.nl.
7.100.145.in-addr.arpa. 5 NS ns1.surfnet.nl.
Authority
The response has no authority records.
Additional
ns1.surfnet.nl. 5 A 192.87.106.101
nsauth1.surfsara.nl. 5 A 145.100.12.242
nsauth2.surfsara.nl. 5 A 145.100.12.243
ns1.surfnet.nl. 5 AAAA 2001:610:1:800a:192:87:106:101
nsauth1.surfsara.nl. 5 AAAA 2001:610:108:2043:0:0:0:242
nsauth2.surfsara.nl. 5 AAAA 2001:610:108:2043:0:0:0:243
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0