Apex domains cannot have CNAME records.
Question and response
Question
dig mobilebulgaria.net. NS
Answer
mobilebulgaria.net. 5 NS ns-765.awsdns-31.net.
mobilebulgaria.net. 5 NS ns-1869.awsdns-41.co.uk.
mobilebulgaria.net. 5 NS ns-254.awsdns-31.com.
mobilebulgaria.net. 5 NS ns-1403.awsdns-47.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0