Apex domains cannot have CNAME records.
Question and response
Question
dig 128.166.216.65.in-addr.arpa. NS
Answer
128.166.216.65.in-addr.arpa. 5 NS ns3.silverpop.com.
128.166.216.65.in-addr.arpa. 5 NS ns2.silverpop.com.
128.166.216.65.in-addr.arpa. 5 NS ns1.silverpop.com.
128.166.216.65.in-addr.arpa. 5 NS ns2.p11.dynect.net.
128.166.216.65.in-addr.arpa. 5 NS ns4.p11.dynect.net.
128.166.216.65.in-addr.arpa. 5 NS ns1.p11.dynect.net.
128.166.216.65.in-addr.arpa. 5 NS ns3.p11.dynect.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0