Apex domains cannot have CNAME records.
Question and response
Question
dig adsl-static.isp.belgacom.be. NS
Answer
adsl-static.isp.belgacom.be. 3 NS ns8.skynet.be.
adsl-static.isp.belgacom.be. 3 NS ns5.skynet.be.
adsl-static.isp.belgacom.be. 3 NS ns6.skynet.be.
adsl-static.isp.belgacom.be. 3 NS ns7.skynet.be.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0