Apex domains cannot have CNAME records.
Question and response
Question
dig nelnet.com. NS
Answer
nelnet.com. 1 NS pdns03.domaincontrol.com.
nelnet.com. 1 NS pdns04.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0