Apex domains cannot have CNAME records.
Question and response
Question
dig pro.io. NS
Answer
pro.io. 5 NS nl.pro.io.
pro.io. 5 NS ch.pro.io.
pro.io. 5 NS p.dnh.net.
Authority
The response has no authority records.
Additional
ch.pro.io. 5 A 109.237.142.19
nl.pro.io. 5 A 176.9.166.219
ch.pro.io. 5 AAAA 2a01:64c0:0:5000:0:0:0:19
nl.pro.io. 5 AAAA 2a01:4f8:fff3:66:0:0:0:219
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0