Apex domains cannot have CNAME records.
Question and response
Question
dig donotspam.de. NS
Answer
donotspam.de. 4 NS ns-863.awsdns-43.net.
donotspam.de. 4 NS ns-1288.awsdns-33.org.
donotspam.de. 4 NS ns-1615.awsdns-09.co.uk.
donotspam.de. 4 NS ns-19.awsdns-02.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0