Apex domains cannot have CNAME records.
Question and response
Question
dig ipv6.he.net. NS
Answer
ipv6.he.net. 5 NS ns4.he.net.
ipv6.he.net. 5 NS ns5.he.net.
ipv6.he.net. 5 NS ns3.he.net.
ipv6.he.net. 5 NS ns2.he.net.
ipv6.he.net. 5 NS ns1.he.net.
Authority
The response has no authority records.
Additional
ns1.HE.NET. 5 A 216.218.130.2
ns2.he.net. 5 A 216.218.131.2
NS3.HE.NET. 5 A 216.218.132.2
ns4.HE.NET. 5 A 216.66.1.2
ns5.HE.NET. 5 A 216.66.80.18
ns1.HE.NET. 5 AAAA 2001:470:100:0:0:0:0:2
ns2.he.net. 5 AAAA 2001:470:200:0:0:0:0:2
NS3.HE.NET. 5 AAAA 2001:470:300:0:0:0:0:2
ns4.HE.NET. 5 AAAA 2001:470:400:0:0:0:0:2
ns5.HE.NET. 5 AAAA 2001:470:500:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0