Apex domains cannot have CNAME records.
Question and response
Question
dig 0-27.204.246.77.in-addr.arpa. NS
Answer
0-27.204.246.77.in-addr.arpa. 4 NS nordic1.dnsnode.net.
0-27.204.246.77.in-addr.arpa. 4 NS nsu.dnsnode.net.
0-27.204.246.77.in-addr.arpa. 4 NS foo-ns.foobar.fi.
0-27.204.246.77.in-addr.arpa. 4 NS bar-ns.foobar.fi.
0-27.204.246.77.in-addr.arpa. 4 NS nsp.dnsnode.net.
Authority
The response has no authority records.
Additional
bar-ns.foobar.fi. 4 A 77.246.200.1
foo-ns.foobar.fi. 4 A 77.246.204.1
bar-ns.foobar.fi. 4 AAAA 2a00:1190:c02d:0:0:0:0:53
foo-ns.foobar.fi. 4 AAAA 2a00:1190:c02d:f00:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0