Apex domains cannot have CNAME records.
Question and response
Question
dig uptime.de. NS
Answer
uptime.de. 4 NS ns04.uptime.de.
uptime.de. 4 NS ns01.uptime.de.
uptime.de. 4 NS ns02.uptime.de.
uptime.de. 4 NS ns03.uptime.de.
Authority
The response has no authority records.
Additional
ns01.uptime.de. 4 A 62.67.60.20
ns02.uptime.de. 4 A 62.67.253.166
ns03.uptime.de. 4 A 46.30.0.8
ns04.uptime.de. 4 A 46.30.0.9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0