diff --git a/lodash.js b/lodash.js
index 0e97f1309..32c9f1f44 100644
--- a/lodash.js
+++ b/lodash.js
@@ -4446,9 +4446,9 @@
 
         var newValue = customizer
           ? customizer(object[key], source[key], key, object, source)
-          : source[key];
+          : undefined;
 
-        assignValue(object, key, newValue);
+        assignValue(object, key, newValue === undefined ? source[key] : newValue);
       }
       return object;
     }
diff --git a/test/test.js b/test/test.js
index 688b63c2d..4f2364465 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1454,7 +1454,7 @@
     QUnit.test('`_.' + methodName + '` should work with a `customizer` that returns `undefined`', function(assert) {
       assert.expect(1);
 
-      var expected = { 'a': undefined };
+      var expected = { 'a': 1 };
       assert.deepEqual(func({}, expected, noop), expected);
     });
   });