Apex domains cannot have CNAME records.
Question and response
Question
dig hkdns.host. NS
Answer
hkdns.host. 5 NS oxygen.ns.hetzner.com.
hkdns.host. 5 NS hydrogen.ns.hetzner.com.
hkdns.host. 5 NS helium.ns.hetzner.de.
Authority
The response has no authority records.
Additional
helium.ns.hetzner.de. 4 A 193.47.99.5
oxygen.ns.hetzner.com. 4 A 88.198.229.192
hydrogen.ns.hetzner.com. 4 A 213.133.100.98
helium.ns.hetzner.de. 4 AAAA 2001:67c:192c:0:0:0:add:5
oxygen.ns.hetzner.com. 4 AAAA 2a01:4f8:0:1:0:0:add:2992
hydrogen.ns.hetzner.com. 4 AAAA 2a01:4f8:0:1:0:0:add:1098
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0