Apex domains cannot have CNAME records.
Question and response
Question
dig swishmail.com. NS
Answer
swishmail.com. 5 NS ns1.dnsimple.com.
swishmail.com. 5 NS ns1.webgoku.com.
swishmail.com. 5 NS ns3.dnsimple.com.
swishmail.com. 5 NS ns2.dnsimple.com.
Authority
The response has no authority records.
Additional
ns1.webgoku.com. 5 A 208.72.58.2
ns1.dnsimple.com. 5 A 162.159.24.4
ns3.dnsimple.com. 5 A 162.159.26.4
ns1.dnsimple.com. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1804
ns3.dnsimple.com. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1a04
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0