Apex domains cannot have CNAME records.
Question and response
Question
dig zee.tuxdns.net. NS
Answer
zee.tuxdns.net. 5 NS ns1.100h.pt.
zee.tuxdns.net. 5 NS ns2.100h.pt.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0