Apex domains cannot have CNAME records.
Question and response
Question
dig static.bluetone.cz. NS
Answer
static.bluetone.cz. 5 NS ns2.bluetone.cz.
static.bluetone.cz. 5 NS ns.bluetone.cz.
static.bluetone.cz. 5 NS ns1.bluetone.cz.
Authority
The response has no authority records.
Additional
ns.bluetone.cz. 5 A 212.158.128.12
ns1.bluetone.cz. 5 A 212.158.128.13
ns2.bluetone.cz. 5 A 85.207.207.75
ns.bluetone.cz. 5 AAAA 2a02:a40:2:0:0:0:0:12
ns1.bluetone.cz. 5 AAAA 2a02:a40:2:0:0:0:0:13
ns2.bluetone.cz. 5 AAAA 2a02:a40:2:200:0:0:2:200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0