Apex domains cannot have CNAME records.
Question and response
Question
dig 215.15.199.in-addr.arpa. NS
Answer
215.15.199.in-addr.arpa. 5 NS ns1.marketo.com.
215.15.199.in-addr.arpa. 5 NS ns2.mktdns.com.
Authority
The response has no authority records.
Additional
ns1.marketo.com. 5 A 199.15.213.1
ns2.mktdns.com. 5 A 199.15.215.252
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0