Apex domains cannot have CNAME records.
Question and response
Question
dig apartments-prague6.com. NS
Answer
apartments-prague6.com. 5 NS ns-741.awsdns-28.net.
apartments-prague6.com. 5 NS ns-216.awsdns-27.com.
apartments-prague6.com. 5 NS ns-1789.awsdns-31.co.uk.
apartments-prague6.com. 5 NS ns-1110.awsdns-10.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0