Apex domains cannot have CNAME records.
Question and response
Question
dig io. NS
Answer
io. 5 NS b0.nic.io.
io. 5 NS a2.nic.io.
io. 5 NS a0.nic.io.
io. 5 NS c0.nic.io.
Authority
The response has no authority records.
Additional
a0.nic.io. 156297 A 65.22.160.17
a2.nic.io. 171224 A 65.22.163.17
b0.nic.io. 165907 A 65.22.161.17
c0.nic.io. 165907 A 65.22.162.17
a0.nic.io. 156297 AAAA 2a01:8840:9e:0:0:0:0:17
a2.nic.io. 171224 AAAA 2a01:8840:a1:0:0:0:0:17
b0.nic.io. 165907 AAAA 2a01:8840:9f:0:0:0:0:17
c0.nic.io. 165907 AAAA 2a01:8840:a0:0:0:0:0:17
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0