Apex domains cannot have CNAME records.
Question and response
Question
dig clients.sappa.info. NS
Answer
clients.sappa.info. 3 NS ns1.sappa.info.
clients.sappa.info. 3 NS ns2.sappa.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0