Apex domains cannot have CNAME records.
Question and response
Question
dig fastxs.net. NS
Answer
fastxs.net. 5 NS ns7.sentia.net.
fastxs.net. 5 NS ns5.sentia.nl.
fastxs.net. 5 NS ns4.sentia.nl.
fastxs.net. 5 NS ns6.sentia.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0