Apex domains cannot have CNAME records.
Question and response
Question
dig hfc.comcastbusiness.net. NS
Answer
hfc.comcastbusiness.net. 5 NS dns105.comcast.net.
hfc.comcastbusiness.net. 5 NS dns104.comcast.net.
hfc.comcastbusiness.net. 5 NS dns103.comcast.net.
hfc.comcastbusiness.net. 5 NS dns102.comcast.net.
hfc.comcastbusiness.net. 5 NS dns101.comcast.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0