Apex domains cannot have CNAME records.
Question and response
Question
dig rubycom.net. NS
Answer
rubycom.net. 3 NS ns1.dnsrubycom.com.
rubycom.net. 3 NS ns4.dnsrubycom.com.
rubycom.net. 3 NS ns3.dnsrubycom.com.
rubycom.net. 3 NS ns2.dnsrubycom.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0