diff --git a/test/collections.js b/test/collections.js
index 1726629eb..499432ac1 100644
--- a/test/collections.js
+++ b/test/collections.js
@@ -77,13 +77,13 @@ $(document).ready(function() {
     ok(ifnull instanceof TypeError, 'handles a null (without inital value) properly');
 
     ok(_.reduce(null, function(){}, 138) === 138, 'handles a null (with initial value) properly');
+    equals(_.reduce([], function(){}, undefined), undefined, 'undefined can be passed as a special case');
+    raises(function() { _.reduce([], function(){}); }, TypeError, 'throws an error for empty arrays with no initial value');
 
-    // Sparse arrays:
-    var sparseArray  = [];
-    sparseArray[100] = 10;
-    sparseArray[200] = 20;
-
-    equals(_.reduce(sparseArray, function(a, b){ return a + b }), 30, 'initially-sparse arrays with no memo');
+    var sparseArray = [];
+    sparseArray[0] = 20;
+    sparseArray[2] = -5;
+    equals(_.reduce(sparseArray, function(a, b){ return a - b }), 25, 'initially-sparse arrays with no memo');
   });
 
   test('collections: reduceRight', function() {
@@ -105,6 +105,14 @@ $(document).ready(function() {
     ok(ifnull instanceof TypeError, 'handles a null (without inital value) properly');
 
     ok(_.reduceRight(null, function(){}, 138) === 138, 'handles a null (with initial value) properly');
+
+    equals(_.reduceRight([], function(){}, undefined), undefined, 'undefined can be passed as a special case');
+    raises(function() { _.reduceRight([], function(){}); }, TypeError, 'throws an error for empty arrays with no initial value');
+
+    var sparseArray = [];
+    sparseArray[0] = 20;
+    sparseArray[2] = -5;
+    equals(_.reduceRight(sparseArray, function(a, b){ return a - b }), -25, 'initially-sparse arrays with no memo');
   });
 
   test('collections: detect', function() {
diff --git a/underscore.js b/underscore.js
index 6e01bbad1..22446234d 100644
--- a/underscore.js
+++ b/underscore.js
@@ -108,7 +108,7 @@
   // **Reduce** builds up a single result from a list of values, aka `inject`,
   // or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
   _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
-    var initial = memo !== void 0;
+    var initial = arguments.length > 2;
     if (obj == null) obj = [];
     if (nativeReduce && obj.reduce === nativeReduce) {
       if (context) iterator = _.bind(iterator, context);
@@ -122,20 +122,22 @@
         memo = iterator.call(context, memo, value, index, list);
       }
     });
-    if (!initial) throw new TypeError("Reduce of empty array with no initial value");
+    if (!initial) throw new TypeError('Reduce of empty array with no initial value');
     return memo;
   };
 
   // The right-associative version of reduce, also known as `foldr`.
   // Delegates to **ECMAScript 5**'s native `reduceRight` if available.
   _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
+    var initial = arguments.length > 2;
     if (obj == null) obj = [];
     if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
       if (context) iterator = _.bind(iterator, context);
-      return memo !== void 0 ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
+      return initial ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
     }
-    var reversed = (_.isArray(obj) ? obj.slice() : _.toArray(obj)).reverse();
-    return _.reduce(reversed, iterator, memo, context);
+    var reversed = _.toArray(obj).reverse();
+    if (context && !initial) iterator = _.bind(iterator, context);
+    return initial ? _.reduce(reversed, iterator, memo, context) : _.reduce(reversed, iterator);
   };
 
   // Return the first value which passes a truth test. Aliased as `detect`.