Apex domains cannot have CNAME records.
Question and response
Question
dig openroaming1.eduroam.org. NS
Answer
openroaming1.eduroam.org. 4 NS glb1-ams1.surfcloud.nl.
openroaming1.eduroam.org. 4 NS glb1-ams2.as1103.net.
openroaming1.eduroam.org. 4 NS glb1-dub1.edu.nl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0