Apex domains cannot have CNAME records.
Question and response
Question
dig namesrv.io. NS
Answer
namesrv.io. 2 NS ns3.namesrv.io.
namesrv.io. 2 NS ns1.namesrv.io.
namesrv.io. 2 NS ns2.namesrv.io.
Authority
The response has no authority records.
Additional
ns1.namesrv.io. 2 A 109.237.142.8
ns2.namesrv.io. 2 A 148.251.254.105
ns3.namesrv.io. 2 A 109.237.143.8
ns1.namesrv.io. 2 AAAA 2a01:64c0:0:5000:0:0:0:8
ns2.namesrv.io. 2 AAAA 2a01:4f8:2b02:a88:0:0:0:105
ns3.namesrv.io. 2 AAAA 2a02:2b80:101:5000:0:0:0:8
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0