Apex domains cannot have CNAME records.
Question and response
Question
dig sv. NS
Answer
sv. 5 NS dns-ext.nic.cr.
sv. 5 NS sir.red.sv.
sv. 5 NS ns.dns.br.
sv. 5 NS p.ns.sv.
sv. 5 NS a.lactld.org.
Authority
The response has no authority records.
Additional
a.lactld.org. 4 A 200.0.68.10
p.ns.sv. 143251 A 204.61.216.112
sir.red.sv. 30043 A 131.100.140.162
a.lactld.org. 4 AAAA 2801:14:a000:0:0:0:0:10
p.ns.sv. 143251 AAAA 2001:500:14:6112:ad:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0