Apex domains cannot have CNAME records.
Question and response
Question
dig su. NS
Answer
su. 4 NS b.dns.ripn.net.
su. 4 NS e.dns.ripn.net.
su. 4 NS a.dns.ripn.net.
su. 4 NS d.dns.ripn.net.
su. 4 NS f.dns.ripn.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0