Apex domains cannot have CNAME records.
Question and response
Question
dig us.planet.net. NS
Answer
us.planet.net. 5 NS ns-1347.awsdns-40.org.
us.planet.net. 5 NS ns-347.awsdns-43.com.
us.planet.net. 5 NS ns-552.awsdns-05.net.
us.planet.net. 5 NS ns-1691.awsdns-19.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0