diff --git a/lib/mongo/collection.rb b/lib/mongo/collection.rb
index 315982b..0be45e0 100644
--- a/lib/mongo/collection.rb
+++ b/lib/mongo/collection.rb
@@ -565,7 +565,7 @@ module Mongo
     def map_reduce(map, reduce, opts={})
       map    = BSON::Code.new(map) unless map.is_a?(BSON::Code)
       reduce = BSON::Code.new(reduce) unless reduce.is_a?(BSON::Code)
-      raw    = opts[:raw]
+      raw    = opts.delete(:raw)
 
       hash = BSON::OrderedHash.new
       hash['mapreduce'] = self.name