Apex domains cannot have CNAME records.
Question and response
Question
dig u2d.de. NS
Answer
u2d.de. 5 NS ns-1956.awsdns-52.co.uk.
u2d.de. 5 NS ns-1496.awsdns-59.org.
u2d.de. 5 NS ns-cloud-a2.googledomains.com.
u2d.de. 5 NS ns-cloud-a1.googledomains.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0