Apex domains cannot have CNAME records.
Question and response
Question
dig mydnsservice.com. NS
Answer
mydnsservice.com. 5 NS admi292808.mars.orderbox-dns.com.
mydnsservice.com. 5 NS admi292808.venus.orderbox-dns.com.
mydnsservice.com. 5 NS admi292808.earth.orderbox-dns.com.
mydnsservice.com. 5 NS admi292808.mercury.orderbox-dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0