Apex domains cannot have CNAME records.
Question and response
Question
dig mktoweb.com. NS
Answer
mktoweb.com. 2 NS ns2.mktdns.com.
mktoweb.com. 2 NS ns1.marketo.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0