Apex domains cannot have CNAME records.
Question and response
Question
dig bigcommerce.net. NS
Answer
bigcommerce.net. 5 NS dns2.p01.nsone.net.
bigcommerce.net. 5 NS dns1.p01.nsone.net.
bigcommerce.net. 5 NS ns-cloud-a2.googledomains.com.
bigcommerce.net. 5 NS ns-cloud-a1.googledomains.com.
Authority
The response has no authority records.
Additional
dns1.p01.nsone.net. 5 A 198.51.44.1
dns2.p01.nsone.net. 5 A 198.51.45.1
ns-cloud-a1.googledomains.com. 3 A 216.239.32.106
ns-cloud-a2.googledomains.com. 3 A 216.239.34.106
dns1.p01.nsone.net. 5 AAAA 2620:4d:4000:6259:7:1:0:1
dns2.p01.nsone.net. 5 AAAA 2a00:edc0:6259:7:1:0:0:2
ns-cloud-a1.googledomains.com. 3 AAAA 2001:4860:4802:32:0:0:0:6a
ns-cloud-a2.googledomains.com. 3 AAAA 2001:4860:4802:34:0:0:0:6a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0