Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-21.com. NS
Answer
awsdns-21.com. 5 NS g-ns-597.awsdns-21.com.
awsdns-21.com. 5 NS g-ns-1749.awsdns-21.com.
awsdns-21.com. 5 NS g-ns-22.awsdns-21.com.
awsdns-21.com. 5 NS g-ns-1173.awsdns-21.com.
Authority
The response has no authority records.
Additional
g-ns-22.awsdns-21.com. 5 A 205.251.192.22
g-ns-597.awsdns-21.com. 5 A 205.251.194.85
g-ns-1173.awsdns-21.com. 5 A 205.251.196.149
g-ns-1749.awsdns-21.com. 5 A 205.251.198.213
g-ns-22.awsdns-21.com. 5 AAAA 2600:9000:5300:1600:0:0:0:1
g-ns-597.awsdns-21.com. 5 AAAA 2600:9000:5302:5500:0:0:0:1
g-ns-1173.awsdns-21.com. 5 AAAA 2600:9000:5304:9500:0:0:0:1
g-ns-1749.awsdns-21.com. 5 AAAA 2600:9000:5306:d500:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0