Apex domains cannot have CNAME records.
Question and response
Question
dig um40.pools.vodafone-ip.de. NS
Answer
um40.pools.vodafone-ip.de. 3 NS ns2.arcor-ip.de.
um40.pools.vodafone-ip.de. 3 NS ns1.arcor-ip.de.
um40.pools.vodafone-ip.de. 3 NS ns3.arcor-ip.de.
Authority
The response has no authority records.
Additional
ns1.arcor-ip.de. 3 A 145.253.2.19
ns2.arcor-ip.de. 3 A 145.253.2.80
ns3.arcor-ip.de. 3 A 145.253.3.171
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0