Apex domains cannot have CNAME records.
Question and response
Question
dig dns2.dnsclusterhost.com. NS
Answer
dns2.dnsclusterhost.com. 5 NS ns-uk.topdns.com.
dns2.dnsclusterhost.com. 5 NS ns-canada.topdns.com.
dns2.dnsclusterhost.com. 5 NS ns-usa.topdns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0