Mongo Assertion Error With MapReduce

gistfile1.rb

class SomeDocument
  include MongoMapper::Document

  # Other stuff...

  def self.tag_cloud_map
    <<-MAP
      function() {
        this.tags.forEach(function(tag) {
          emit(tag, 1);
        });
      }
    MAP
  end
  
  def self.tag_cloud_reduce
    <<-REDUCE
      function(key, values) {
        var count = 0;
        
        values.forEach(function(value) {
          count += 1;
        });
        return count;
      }
    REDUCE
  end
  
  def self.tag_cloud_build(opts)
    self.collection.map_reduce(self.tag_cloud_map, self.tag_cloud_reduce, opts)
  end
  
  def self.tag_cloud(opts={})
    opts.merge({
      :out    => {:inline => true},
      :raw    => true 
    })
    self.tag_cloud_build(opts).find()
  end

end