Apex domains cannot have CNAME records.
Question and response
Question
dig postal.k.io. NS
Answer
postal.k.io. 5 NS green-cheetah.katapultdns.com.
postal.k.io. 5 NS dazzling-vista.katapultdns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0