Apex domains cannot have CNAME records.
Question and response
Question
dig rsd-series.com. NS
Answer
rsd-series.com. 5 NS ns-413.awsdns-51.com.
rsd-series.com. 5 NS ns-249.awsdns-31.com.
rsd-series.com. 5 NS ns-2042.awsdns-63.co.uk.
rsd-series.com. 5 NS ns-1676.awsdns-17.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0