Apex domains cannot have CNAME records.
Question and response
Question
dig gtdsl.com. NS
Answer
gtdsl.com. 5 NS ns-1430.awsdns-50.org.
gtdsl.com. 5 NS ns-44.awsdns-05.com.
gtdsl.com. 5 NS ns-1074.awsdns-06.org.
gtdsl.com. 5 NS ns-1418.awsdns-49.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0