Apex domains cannot have CNAME records.
Question and response
Question
dig 217.166.192.in-addr.arpa.teredo.pl. NS
Answer
217.166.192.in-addr.arpa.teredo.pl. 5 NS ns2.webh.pro.
217.166.192.in-addr.arpa.teredo.pl. 5 NS ns1.webh.pro.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0