Apex domains cannot have CNAME records.
Question and response
Question
dig 208-28.2.20.62.in-addr.arpa. NS
Answer
208-28.2.20.62.in-addr.arpa. 5 NS gatekeeper1.kgm.se.
208-28.2.20.62.in-addr.arpa. 5 NS goofy.dcp.se.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0