Apex domains cannot have CNAME records.
Question and response
Question
dig digicert.com. NS
Answer
digicert.com. 2 NS pdns196.ultradns.org.
digicert.com. 2 NS ns20.digicertdns.com.
digicert.com. 2 NS ns21.digicertdns.com.
digicert.com. 2 NS ns22.digicertdns.com.
digicert.com. 2 NS ns23.digicertdns.net.
digicert.com. 2 NS ns24.digicertdns.net.
digicert.com. 2 NS ns25.digicertdns.net.
digicert.com. 2 NS pdns196.ultradns.biz.
digicert.com. 2 NS pdns196.ultradns.com.
digicert.com. 2 NS pdns196.ultradns.net.
Authority
The response has no authority records.
Additional
ns20.digicertdns.com. 2 A 208.94.148.223
ns21.digicertdns.com. 2 A 208.80.124.223
ns22.digicertdns.com. 2 A 208.80.126.223
ns23.digicertdns.net. 2 A 208.80.125.223
ns24.digicertdns.net. 2 A 208.80.127.223
ns25.digicertdns.net. 2 A 208.94.149.223
ns20.digicertdns.com. 2 AAAA 2600:1800:10:0:0:0:1:1f
ns21.digicertdns.com. 2 AAAA 2600:1801:11:0:0:0:1:1f
ns22.digicertdns.com. 2 AAAA 2600:1802:12:0:0:0:1:1f
ns23.digicertdns.net. 2 AAAA 2600:1801:13:0:0:0:1:1f
ns24.digicertdns.net. 2 AAAA 2600:1802:14:0:0:0:1:1f
ns25.digicertdns.net. 2 AAAA 2600:1800:15:0:0:0:1:1f
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0