Apex domains cannot have CNAME records.
Question and response
Question
dig serverdomain.org. NS
Answer
serverdomain.org. 4 NS ns2.nshost2.de.
serverdomain.org. 4 NS ns1.nshost2.de.
serverdomain.org. 4 NS ns3.nshost2.net.
Authority
The response has no authority records.
Additional
ns1.nshost2.de. 4 A 89.107.184.116
ns2.nshost2.de. 4 A 89.107.185.20
ns3.nshost2.net. 4 A 152.53.140.148
ns3.nshost2.net. 4 AAAA 2a0a:4cc0:c1:660:880a:29ff:feb2:ada5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0