Apex domains cannot have CNAME records.
Question and response
Question
dig 136.in-addr.arpa. NS
Answer
136.in-addr.arpa. 5 NS u.arin.net.
136.in-addr.arpa. 5 NS arin.authdns.ripe.net.
136.in-addr.arpa. 5 NS y.arin.net.
136.in-addr.arpa. 5 NS z.arin.net.
136.in-addr.arpa. 5 NS r.arin.net.
136.in-addr.arpa. 5 NS x.arin.net.
Authority
The response has no authority records.
Additional
r.arin.net. 1 A 199.180.180.63
u.arin.net. 1 A 204.61.216.50
x.arin.net. 1 A 199.180.180.63
y.arin.net. 1 A 192.82.134.30
z.arin.net. 1 A 199.180.180.63
arin.authdns.ripe.net. 1 A 193.0.9.10
r.arin.net. 1 AAAA 2001:500:f0:0:0:0:0:63
u.arin.net. 1 AAAA 2001:500:14:6050:ad:0:0:1
x.arin.net. 1 AAAA 2001:500:f0:0:0:0:0:63
y.arin.net. 1 AAAA 2001:500:127:0:0:0:0:30
z.arin.net. 1 AAAA 2001:500:f0:0:0:0:0:63
arin.authdns.ripe.net. 1 AAAA 2a13:27c0:30:0:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0