Apex domains cannot have CNAME records.
Question and response
Question
dig cleanport.nl. NS
Answer
cleanport.nl. 5 NS edns109.ultradns.biz.
cleanport.nl. 5 NS edns109.ultradns.com.
cleanport.nl. 5 NS edns109.ultradns.net.
cleanport.nl. 5 NS edns109.ultradns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0