Apex domains cannot have CNAME records.
Question and response
Question
dig clicktools.com. NS
Answer
clicktools.com. 2 NS ns.rackspace.com.
clicktools.com. 2 NS ns2.rackspace.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0