Apex domains cannot have CNAME records.
Question and response
Question
dig ffdev.cz. NS
Answer
ffdev.cz. 2 NS ns3.websupport.eu.
ffdev.cz. 2 NS ns2.websupport.cz.
ffdev.cz. 2 NS ns1.websupport.cz.
Authority
The response has no authority records.
Additional
ns1.websupport.cz. 2 A 45.10.97.11
ns2.websupport.cz. 2 A 185.71.159.12
ns3.websupport.eu. 2 A 45.10.97.13
ns1.websupport.cz. 2 AAAA 2a10:bbc0:0:0:0:0:0:11
ns2.websupport.cz. 2 AAAA 2a02:250:3:0:0:0:0:12
ns3.websupport.eu. 2 AAAA 2a10:bbc0:0:0:0:0:0:13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0