Group by example in MongoMapper

Ruby.rb

class Tweet
  include MongoMapper::Document
  
  key :id_str, String
  key :from_user_id, Integer

  belongs_to :track_item  
  
  def self.grouped_by(column, options = {})
    map_function = "function() { emit( this.#{column}, 1); }"
    
    # put your logic here (not needed in my case)
    reduce_function = %Q( function(key, values) { 
      return true;
    })
    
    collection.map_reduce(map_function, reduce_function, options)
  end
end

Tweet.grouped_by(:from_user_id, :query => {:track_item_id => track_item.id}).find().count

I get what you're doing there but I get:
The error occurred while evaluating nil.last
/Users/mmitchell/.rbenv/versions/ree-1.8.7-2011.03/lib/ruby/gems/1.8/gems/mongo-1.8.2/lib/mongo/util/support.rb:83:in `secondary_ok?'

Any ideas on this?
Many thanks

undefined method `last' for nil:NilClass

I get this error when I try and run map reduce, any idea why?

I get the same error.