Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000004.tangocard.com. NS
Answer
_spf0000004.tangocard.com. 5 NS ns-1956.awsdns-52.co.uk.
_spf0000004.tangocard.com. 5 NS ns-1400.awsdns-47.org.
_spf0000004.tangocard.com. 5 NS ns-334.awsdns-41.com.
_spf0000004.tangocard.com. 5 NS ns-775.awsdns-32.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0