Apex domains cannot have CNAME records.
Question and response
Question
dig f5s.de. NS
Answer
f5s.de. 5 NS ns.inwx.de.
f5s.de. 5 NS ns2.inwx.de.
f5s.de. 5 NS ns3.inwx.eu.
Authority
The response has no authority records.
Additional
ns.inwx.de. 3 A 192.174.68.104
ns2.inwx.de. 3 A 176.97.158.104
ns3.inwx.eu. 3 A 45.87.158.53
ns.inwx.de. 3 AAAA 2001:67c:1bc:0:0:0:0:104
ns2.inwx.de. 3 AAAA 2001:67c:10b8:0:0:0:0:104
ns3.inwx.eu. 3 AAAA 2a02:d500:0:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0