_prefixes.he.net has no CNAME records.
Question and response
Question
dig _prefixes.he.net. NS
Answer
The response has no answer records.
Authority
he.net. 2356 SOA ns1.he.net. hostmaster.he.net. 202602250 10800 1800 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0