diff --git a/lib/bson/types/objectid.rb b/lib/bson/types/objectid.rb
index edb7ff3..30b179b 100644
--- a/lib/bson/types/objectid.rb
+++ b/lib/bson/types/objectid.rb
@@ -140,7 +140,7 @@ module BSON
     # but lacks an ObjectID type, this JSON format encodes the type using an $oid key.
     #
     # @return [String] the object id represented as MongoDB extended JSON.
-    def to_json(escaped=false)
+    def to_json(*a)
       "{\"$oid\": \"#{to_s}\"}"
     end
 
diff --git a/test/mongo_bson/json_test.rb b/test/mongo_bson/json_test.rb
new file mode 100644
index 0000000..441064b
--- /dev/null
+++ b/test/mongo_bson/json_test.rb
@@ -0,0 +1,16 @@
+require 'test/test_helper'
+require 'rubygems'
+require 'json'
+
+class JSONTest < Test::Unit::TestCase
+
+  include Mongo
+  include BSON
+
+  def test_object_id_as_json
+    id = ObjectID.new
+    obj = {'_id' => id}
+    assert_equal "{\"_id\":{\"$oid\": \"#{id.to_s}\"}}", obj.to_json
+  end
+
+end