Apex domains cannot have CNAME records.
Question and response
Question
dig sec-ns.net. NS
Answer
sec-ns.net. 5 NS n.sec-ns.net.
sec-ns.net. 5 NS a.prim-ns.de.
sec-ns.net. 5 NS m.sec-ns.de.
sec-ns.net. 5 NS c.sec-ns.de.
sec-ns.net. 5 NS a.sec-ns.net.
Authority
The response has no authority records.
Additional
a.sec-ns.net. 5 A 195.180.210.17
a.prim-ns.de. 5 A 194.163.252.5
c.sec-ns.de. 5 A 195.180.210.33
n.sec-ns.net. 5 A 54.37.228.71
a.prim-ns.de. 5 AAAA 2a00:74c0:10:203:0:0:53:1
c.sec-ns.de. 5 AAAA 2a00:74c0:10:201:0:0:53:1
n.sec-ns.net. 5 AAAA 2001:41d0:302:2200:0:0:0:45a6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0