Error with geo_code with Mongoosastic and Elasticsearch 1.3.1

I'm getting this error when I index my models with the new version 1.3.1. Any ideas why?

{ error: 'RemoteTransportException[...][index]]; nested: MapperParsingException[failed to parse]; nested: ElasticsearchParseException[field must be either \'lat\', \'lon\' or \'geohash\']; '

My mapping looks like this:

    location:
      geo_point:
        type       : String
        es_type    : 'geo_point'
        es_lat_lon : true
      lat :
        type: Number
      lon :
        type: Number