Apex domains cannot have CNAME records.
Question and response
Question
dig telefonica.de. NS
Answer
telefonica.de. 5 NS ns2.telefonica.de.
telefonica.de. 5 NS ns1.telefonica.de.
telefonica.de. 5 NS ns3.telefonica.de.
Authority
The response has no authority records.
Additional
ns1.telefonica.de. 5 A 62.53.237.12
ns2.telefonica.de. 5 A 213.20.172.204
ns3.telefonica.de. 5 A 195.71.243.188
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0