Apex domains cannot have CNAME records.
Question and response
Question
dig nameserver300.coolcom.com. NS
Answer
nameserver300.coolcom.com. 5 NS ns77.coolcom.com.
nameserver300.coolcom.com. 5 NS ns777.coolcom.com.
nameserver300.coolcom.com. 5 NS ns7.coolcom.com.
Authority
The response has no authority records.
Additional
ns7.coolcom.com. 5 A 72.249.68.145
ns77.coolcom.com. 5 A 209.142.64.139
ns777.coolcom.com. 5 A 165.140.159.117
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0