Apex domains cannot have CNAME records.
Question and response
Question
dig rev.dnainternet.fi. NS
Answer
rev.dnainternet.fi. 5 NS ns5.dnaip.fi.
rev.dnainternet.fi. 5 NS ns4.dnaip.fi.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0