Apex domains cannot have CNAME records.
Question and response
Question
dig tmt.de. NS
Answer
tmt.de. 2 NS ns0.tmt.de.
tmt.de. 2 NS ns2.tmt.de.
tmt.de. 2 NS ns1.tmt.de.
tmt.de. 2 NS ns5.tmt.de.
tmt.de. 2 NS ns3.tmt.de.
Authority
The response has no authority records.
Additional
ns0.tmt.de. 2 A 217.145.97.43
ns1.tmt.de. 2 A 145.239.31.235
ns2.tmt.de. 2 A 188.166.102.15
ns3.tmt.de. 2 A 95.216.148.1
ns5.tmt.de. 2 A 88.209.63.250
ns0.tmt.de. 2 AAAA 2001:4dc8:100:3:0:0:97:43
ns1.tmt.de. 2 AAAA 2001:41d0:601:2000:0:0:0:1bcc
ns2.tmt.de. 2 AAAA 2a03:b0c0:2:d0:0:0:d12:e001
ns3.tmt.de. 2 AAAA 2a01:4f9:c010:c0f:0:0:0:1
ns5.tmt.de. 2 AAAA 2001:4dc8:100:0:216:3eff:fec6:3095
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0