Apex domains cannot have CNAME records.
Question and response
Question
dig customer.alter.net. NS
Answer
customer.alter.net. 2 NS auth210.ns.uu.net.
customer.alter.net. 2 NS auth200.ns.uu.net.
customer.alter.net. 2 NS auth02.ns.uu.net.
customer.alter.net. 2 NS auth51.ns.uu.net.
Authority
The response has no authority records.
Additional
auth200.ns.uu.net. 2 A 195.129.12.82
auth210.ns.uu.net. 2 A 195.129.12.74
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0