I've posted this issue about 3 weeks ago on the solr-user mailing list here:
Basically, when using the json facet api on an enum field, like this:
where state is an enum, i get an error:
The full example can be found here: https://gist.github.com/xou/33fd7aab7afe3cebb28b
And a quick-and-dirty patch can be found here: http://bildungsresistenz.de/tmp.h/0001-Added-support-for-facetting-on-enum-types.patch
I'm not sure whether the patch is the right way to approach this; if it goes in the right direction i can definitely try to clean it up a bit more.