Apex domains cannot have CNAME records.
Question and response
Question
dig 143.239.194.in-addr.arpa. NS
Answer
143.239.194.IN-ADDR.ARPA. 2 NS auth05.ns.tdc.net.
143.239.194.IN-ADDR.ARPA. 2 NS auth06.ns.tdc.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0