Apex domains cannot have CNAME records.
Question and response
Question
dig proserver1.com. NS
Answer
proserver1.com. 3 NS dns2.proserver1.at.
proserver1.com. 3 NS dns3.proserver1.at.
proserver1.com. 3 NS dns1.proserver1.at.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0