Apex domains cannot have CNAME records.
Question and response
Question
dig dkim.us.fireeyecloud.com. NS
Answer
dkim.us.fireeyecloud.com. 5 NS ns-1547.awsdns-01.co.uk.
dkim.us.fireeyecloud.com. 5 NS ns-1152.awsdns-16.org.
dkim.us.fireeyecloud.com. 5 NS ns-492.awsdns-61.com.
dkim.us.fireeyecloud.com. 5 NS ns-966.awsdns-56.net.
Authority
The response has no authority records.
Additional
ns-492.awsdns-61.com. 1 A 205.251.193.236
ns-966.awsdns-56.net. 1 A 205.251.195.198
ns-1152.awsdns-16.org. 1 A 205.251.196.128
ns-1547.awsdns-01.co.uk. 1 A 205.251.198.11
ns-492.awsdns-61.com. 1 AAAA 2600:9000:5301:ec00:0:0:0:1
ns-966.awsdns-56.net. 1 AAAA 2600:9000:5303:c600:0:0:0:1
ns-1152.awsdns-16.org. 1 AAAA 2600:9000:5304:8000:0:0:0:1
ns-1547.awsdns-01.co.uk. 1 AAAA 2600:9000:5306:b00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0