Apex domains cannot have CNAME records.
Question and response
Question
dig a-v1.mx.microsoft. NS
Answer
a-v1.mx.microsoft. 5 NS ns1-83.azure-dns.com.
a-v1.mx.microsoft. 5 NS ns3-83.azure-dns.org.
a-v1.mx.microsoft. 5 NS ns2-83.azure-dns.net.
a-v1.mx.microsoft. 5 NS ns4-83.azure-dns.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0