Apex domains cannot have CNAME records.
Question and response
Question
dig 18.121.IN-ADDR.ARPA. NS
Answer
18.121.IN-ADDR.ARPA. 2 NS dns3.hbidc.net.
18.121.IN-ADDR.ARPA. 2 NS dns4.hbidc.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0