Apex domains cannot have CNAME records.
Question and response
Question
dig ns1.iscg.net. NS
Answer
ns1.iscg.net. 5 NS dns3.iscgnet.com.
ns1.iscg.net. 5 NS dns2.iscgnet.com.
ns1.iscg.net. 5 NS dns1.iscgnet.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0