Apex domains cannot have CNAME records.
Question and response
Question
dig tutum.co. NS
Answer
tutum.co. 5 NS ns-106-a.gandi.net.
tutum.co. 5 NS ns-220-b.gandi.net.
tutum.co. 5 NS ns-198-c.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0