Apex domains cannot have CNAME records.
Question and response
Question
dig ipv6.zsttk.net. NS
Answer
ipv6.zsttk.net. 5 NS ns1.zsttk.net.
ipv6.zsttk.net. 5 NS ns2.zsttk.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0