Apex domains cannot have CNAME records.
Question and response
Question
dig 250.185.89.in-addr.arpa. NS
Answer
250.185.89.in-addr.arpa. 4 NS ns1.master.cz.
250.185.89.in-addr.arpa. 4 NS ns2.master.cz.
250.185.89.in-addr.arpa. 4 NS ns3.master.cz.
Authority
The response has no authority records.
Additional
ns1.master.cz. 4 A 81.31.33.23
ns2.master.cz. 4 A 81.31.33.6
ns3.master.cz. 4 A 82.209.3.134
ns1.master.cz. 4 AAAA 2a01:430:100:16:0:0:0:2
ns2.master.cz. 4 AAAA 2a01:430:200:b:0:0:0:2
ns3.master.cz. 4 AAAA 2a00:ca8:8000:0:0:0:0:c0de
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0