diff --git a/lib/mongo.rb b/lib/mongo.rb
index 1e386a5..5214d0a 100644
--- a/lib/mongo.rb
+++ b/lib/mongo.rb
@@ -21,9 +21,10 @@ $:.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))
 require 'mongo/version'
 
 module Mongo
-  ASCENDING  =  1
-  DESCENDING = -1
-  GEO2D      = '2d'
+  ASCENDING   =  1
+  DESCENDING  = -1
+  GEO2D       = '2d'
+  GEOHAYSTACK = 'geoHaystack'
 
   DEFAULT_MAX_BSON_SIZE = 4 * 1024 * 1024
 
diff --git a/lib/mongo/collection.rb b/lib/mongo/collection.rb
index 8f70eb4..b5b98a5 100644
--- a/lib/mongo/collection.rb
+++ b/lib/mongo/collection.rb
@@ -888,7 +888,7 @@ module Mongo
         field_spec[spec.to_s] = 1
       elsif spec.is_a?(Array) && spec.all? {|field| field.is_a?(Array) }
         spec.each do |f|
-          if [Mongo::ASCENDING, Mongo::DESCENDING, Mongo::GEO2D].include?(f[1])
+          if [Mongo::ASCENDING, Mongo::DESCENDING, Mongo::GEO2D, Mongo::GEOHAYSTACK].include?(f[1])
             field_spec[f[0].to_s] = f[1]
           else
             raise MongoArgumentError, "Invalid index field #{f[1].inspect}; " +