Apex domains cannot have CNAME records.
Question and response
Question
dig ns03.com. NS
Answer
ns03.com. 3 NS ns04.com.
ns03.com. 3 NS ns02.com.
ns03.com. 3 NS ns03.com.
ns03.com. 3 NS ns01.com.
ns03.com. 3 NS ns05.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0