helium.ns.hetzner.com has no CNAME records.
Question and response
Question
dig helium.ns.hetzner.com. NS
Answer
The response has no answer records.
Authority
hetzner.com. 7200 SOA ns1.your-server.de. postmaster.your-server.de. 2026030600 14400 10800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0