Apex domains cannot have CNAME records.
Question and response
Question
dig us266.siteground.us. NS
Answer
us266.siteground.us. 5 NS ns2.us266.siteground.us.
us266.siteground.us. 5 NS ns1.us266.siteground.us.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0