Apex domains cannot have CNAME records.
Question and response
Question
dig 0-254.19.233.89.in-addr.arpa. NS
Answer
0-254.19.233.89.in-addr.arpa. 5 NS ns07.sac-it.dk.
0-254.19.233.89.in-addr.arpa. 5 NS ns08.sac-it.dk.
Authority
The response has no authority records.
Additional
ns07.sac-it.dk. 5 A 85.235.243.63
ns08.sac-it.dk. 5 A 128.76.60.63
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0