Apex domains cannot have CNAME records.
Question and response
Question
dig stringop.de.com. NS
Answer
stringop.de.com. 5 NS mallory.ns.cloudflare.com.
stringop.de.com. 5 NS brad.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
brad.ns.cloudflare.com. 2 A 108.162.193.105
brad.ns.cloudflare.com. 2 A 172.64.33.105
brad.ns.cloudflare.com. 2 A 173.245.59.105
mallory.ns.cloudflare.com. 2 A 108.162.194.83
mallory.ns.cloudflare.com. 2 A 162.159.38.83
mallory.ns.cloudflare.com. 2 A 172.64.34.83
brad.ns.cloudflare.com. 2 AAAA 2606:4700:58:0:0:0:adf5:3b69
brad.ns.cloudflare.com. 2 AAAA 2803:f800:50:0:0:0:6ca2:c169
brad.ns.cloudflare.com. 2 AAAA 2a06:98c1:50:0:0:0:ac40:2169
mallory.ns.cloudflare.com. 2 AAAA 2606:4700:50:0:0:0:a29f:2653
mallory.ns.cloudflare.com. 2 AAAA 2803:f800:50:0:0:0:6ca2:c253
mallory.ns.cloudflare.com. 2 AAAA 2a06:98c1:50:0:0:0:ac40:2253
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0