facet method=enum and uninvertedfield limitations

[Lemke, Michael SZ/HZA-ZSW]

I am running into performance problems with faceted queries.
If I do a 

q=word&facet.field=CONTENT&facet=true&facet.limit=10&facet.mincount=1&facet.method=fc&facet.prefix=a&rows=0

I am getting an exception:
org.apache.solr.common.SolrException: Too many values for UnInvertedField 
faceting on field CONTENT
        at 
org.apache.solr.request.UnInvertedField.uninvert(UnInvertedField.java:384)
        at 
org.apache.solr.request.UnInvertedField.<init>(UnInvertedField.java:178)
        at 
org.apache.solr.request.UnInvertedField.getUnInvertedField(UnInvertedField.java:839)
        ...

I understand it's got something to do with a 24bit limit somewhere
in the code but I don't understand enough of it to be able to construct
a specialized index that can be queried with facet.method=enum.

A stripped down index still doesn't work.  It has exactly one
field CONTENT with 178,000 Terms and ~1 mio documents.  The top
ranking terms according to Luke are

1 413950        CONTENT word1
2 321223        CONTENT word2
3 299036        CONTENT word3
4 276757        CONTENT word4
...

How would we have to strip the index?

Thanks,
Michael