Apex domains cannot have CNAME records.
Question and response
Question
dig mta4.net. NS
Answer
mta4.net. 5 NS dns42.cloudns.net.
mta4.net. 5 NS dns44.cloudns.net.
mta4.net. 5 NS dns41.cloudns.net.
mta4.net. 5 NS dns43.cloudns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0