Apex domains cannot have CNAME records.
Question and response
Question
dig dns-us.st. NS
Answer
dns-us.st. 5 NS ns.bahnhof.net.
dns-us.st. 5 NS ns2.bahnhof.net.
Authority
The response has no authority records.
Additional
ns.bahnhof.net. 5 A 195.178.160.1
ns2.bahnhof.net. 5 A 212.85.64.4
ns.bahnhof.net. 5 AAAA 2001:9b0:0:0:0:0:53:4
ns2.bahnhof.net. 5 AAAA 2001:9b0:0:0:0:0:53:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0