Apex domains cannot have CNAME records.
Question and response
Question
dig mdsl.com. NS
Answer
mdsl.com. 3 NS ns-1777.awsdns-30.co.uk.
mdsl.com. 3 NS ns-603.awsdns-11.net.
mdsl.com. 3 NS ns-360.awsdns-45.com.
mdsl.com. 3 NS ns-1479.awsdns-56.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0