Apex domains cannot have CNAME records.
Question and response
Question
dig mt. NS
Answer
mt. 5 NS f.ns.mt.
mt. 5 NS b.ns.mt.
mt. 5 NS a.ns.mt.
mt. 5 NS p.ns.mt.
Authority
The response has no authority records.
Additional
a.ns.mt. 155994 A 193.188.47.252
b.ns.mt. 166027 A 193.188.34.241
f.ns.mt. 38832 A 192.93.0.4
p.ns.mt. 38832 A 204.61.216.45
f.ns.mt. 38832 AAAA 2001:660:3005:1:0:0:1:2
p.ns.mt. 38832 AAAA 2001:500:14:6045:ad:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0