Apex domains cannot have CNAME records.
Question and response
Question
dig static.chello.sk. NS
Answer
static.chello.sk. 5 NS rns2.upc.biz.
static.chello.sk. 5 NS rns1.upc.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0