Apex domains cannot have CNAME records.
Question and response
Question
dig nic.DO. NS
Answer
nic.do. 4 NS ns4.nic.do.
nic.do. 4 NS ns5.nic.do.
nic.do. 4 NS ns1.nic.do.
nic.do. 4 NS ns2.nic.do.
nic.do. 4 NS ns.nic.do.
nic.do. 4 NS a.lactld.org.
nic.do. 4 NS phloem.uoregon.edu.
Authority
The response has no authority records.
Additional
ns.nic.do. 75212 A 190.113.72.177
ns1.nic.do. 75212 A 190.113.72.178
ns2.nic.do. 75212 A 190.113.65.12
ns4.nic.do. 75212 A 204.61.216.124
ns5.nic.do. 75212 A 190.113.65.13
ns.nic.do. 75212 AAAA 2001:13e0:85d0:106:0:0:0:177
ns1.nic.do. 75212 AAAA 2001:13e0:85d0:106:0:0:0:178
ns2.nic.do. 75212 AAAA 2001:13e0:0:1:0:0:0:12
ns4.nic.do. 75212 AAAA 2001:500:14:6124:ad:0:0:1
ns5.nic.do. 75212 AAAA 2001:13e0:0:1:0:0:0:13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0