diff --git a/lodash.js b/lodash.js
index c5a286afe..c56e9eba9 100644
--- a/lodash.js
+++ b/lodash.js
@@ -841,7 +841,7 @@
         result = result === undefined ? current : (result + current);
       }
     }
-    return result;
+    return length ? result : 0;
   }
 
   /**
@@ -13917,7 +13917,7 @@
     function sum(array) {
       return (array && array.length)
         ? baseSum(array, identity)
-        : undefined;
+        : 0;
     }
 
     /**
@@ -13945,7 +13945,7 @@
     function sumBy(array, iteratee) {
       return (array && array.length)
         ? baseSum(array, getIteratee(iteratee))
-        : undefined;
+        : 0;
     }
 
     /*------------------------------------------------------------------------*/
diff --git a/test/test.js b/test/test.js
index 228e7bba1..d70a5e13a 100644
--- a/test/test.js
+++ b/test/test.js
@@ -12878,6 +12878,15 @@
       var array = [4, 2, 8, 6];
       assert.strictEqual(_.mean(array), 5);
     });
+
+    QUnit.test('should return `NaN` when passing empty `array` values', function(assert) {
+      assert.expect(1);
+
+      var expected = lodashStable.map(empties, alwaysNaN),
+          actual = lodashStable.map(empties, _.mean);
+
+      assert.deepEqual(actual, expected);
+    });
   }());
 
   /*--------------------------------------------------------------------------*/
@@ -18713,10 +18722,10 @@
       assert.strictEqual(_.sum(array), 12);
     });
 
-    QUnit.test('should return `undefined` when passing empty `array` values', function(assert) {
+    QUnit.test('should return `0` when passing empty `array` values', function(assert) {
       assert.expect(1);
 
-      var expected = lodashStable.map(empties, alwaysUndefined),
+      var expected = lodashStable.map(empties, alwaysZero),
           actual = lodashStable.map(empties, _.sum);
 
       assert.deepEqual(actual, expected);