Empty buckets when faceting with JSON API on EnumField or TrieIntField

When using the JSON API, faceting on an EnumField or TrieIntField returns an empty list of buckets.

Steps to reproduce:
1. Define an EnumField.
schema.xml:
<field name="kingdom" type="kingdomEnum" indexed="true" stored="true" required="true" />
<fieldType name="kingdomEnum" class="solr.EnumField" enumsConfig="enumsConfig.xml" enumName="kingdom"/>

enumsConfig.xml:
<?xml version="1.0" ?>
<enumsConfig>
<enum name="kingdom">
<value>plants</value>
<value>metazoa</value>
<value>fungi</value>
</enum>
</enumsConfig>

2. Add documents to core.

3. Faceting over the EnumField returns an empty list of buckets:
$ curl http://localhost:8983/solr/myCore/query -d '
{
query: ":",
facet : {
kingdom :

}
}'
{ ...
"facets":{
"count":17,
"kingdom":

}}

The same issue happens with TrieIntField. Using old-style facets works, this only happens with the new JSON API.

I tested changing the field to a string and I get the expected results.