Apex domains cannot have CNAME records.
Question and response
Question
dig opennameserver.org. NS
Answer
opennameserver.org. 5 NS ns1.domainers.de.
opennameserver.org. 5 NS ns2.domainers.de.
opennameserver.org. 5 NS ns3.domainers.de.
opennameserver.org. 5 NS ns4.domainers.de.
Authority
The response has no authority records.
Additional
ns1.domainers.de. 3 A 95.130.16.246
ns2.domainers.de. 3 A 95.130.17.218
ns3.domainers.de. 3 A 217.146.22.194
ns4.domainers.de. 3 A 144.76.91.137
ns1.domainers.de. 3 AAAA 2a02:2940:0:a0d1:0:0:0:246
ns2.domainers.de. 3 AAAA 2a02:2940:0:a0d2:0:0:0:218
ns4.domainers.de. 3 AAAA 2a01:4f8:192:2187:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0