Apex domains cannot have CNAME records.
Question and response
Question
dig register.bg. NS
Answer
register.bg. 5 NS ns3.register.bg.
register.bg. 5 NS ns.digsys.bg.
register.bg. 5 NS ns2.digsys.bg.
register.bg. 5 NS ns.register.bg.
register.bg. 5 NS ns2.register.bg.
register.bg. 5 NS ns4.register.bg.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0