Apex domains cannot have CNAME records.
Question and response
Question
dig dnsimple.com. NS
Answer
dnsimple.com. 5 NS ns4.dnsimple-edge.org.
dnsimple.com. 5 NS ns2.dnsimple-edge.net.
dnsimple.com. 5 NS ns1.dnsimple.com.
dnsimple.com. 5 NS ns3.dnsimple.com.
Authority
The response has no authority records.
Additional
ns1.dnsimple.com. 5 A 162.159.24.4
ns2.dnsimple-edge.net. 5 A 199.247.153.53
ns3.dnsimple.com. 5 A 162.159.26.4
ns4.dnsimple-edge.org. 5 A 199.247.155.53
ns1.dnsimple.com. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1804
ns2.dnsimple-edge.net. 5 AAAA 2620:111:8005:0:0:0:0:53
ns3.dnsimple.com. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1a04
ns4.dnsimple-edge.org. 5 AAAA 2620:111:8007:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0