Apex domains cannot have CNAME records.
Question and response
Question
dig mx.skynet.be. NS
Answer
mx.skynet.be. 5 NS ns2.skynet.be.
mx.skynet.be. 5 NS ns1.skynet.be.
mx.skynet.be. 5 NS ns4.skynet.be.
mx.skynet.be. 5 NS ns3.skynet.be.
Authority
The response has no authority records.
Additional
ns1.skynet.be. 5 A 195.238.3.17
ns2.skynet.be. 5 A 195.238.3.18
ns3.skynet.be. 5 A 195.238.25.138
ns4.skynet.be. 5 A 195.238.25.139
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0