diff --git a/moment.js b/moment.js
index 4858e241bb..fd84245790 100644
--- a/moment.js
+++ b/moment.js
@@ -298,9 +298,6 @@
             seconds = normalizedInput.second || 0,
             milliseconds = normalizedInput.millisecond || 0;
 
-        // store reference to input for deterministic cloning
-        this._input = duration;
-
         // representation for dateAddRemove
         this._milliseconds = +milliseconds +
             seconds * 1e3 + // 1000
@@ -1559,9 +1556,7 @@
 
     // duration
     moment.duration = function (input, key) {
-        var isDuration = moment.isDuration(input),
-            isNumber = (typeof input === 'number'),
-            duration = (isDuration ? input._input : (isNumber ? {} : input)),
+        var duration = input,
             // matching against regexp is expensive, do it on demand
             match = null,
             sign,
@@ -1570,7 +1565,14 @@
             timeEmpty,
             dateTimeEmpty;
 
-        if (isNumber) {
+        if (moment.isDuration(input)) {
+            duration = {
+                ms: input._milliseconds,
+                d: input._days,
+                M: input._months
+            };
+        } else if (typeof input === 'number') {
+            duration = {};
             if (key) {
                 duration[key] = input;
             } else {
@@ -1609,7 +1611,7 @@
 
         ret = new Duration(duration);
 
-        if (isDuration && input.hasOwnProperty('_lang')) {
+        if (moment.isDuration(input) && input.hasOwnProperty('_lang')) {
             ret._lang = input._lang;
         }
 
diff --git a/test/moment/duration.js b/test/moment/duration.js
index d5e1d4baf4..e0377054ec 100644
--- a/test/moment/duration.js
+++ b/test/moment/duration.js
@@ -130,12 +130,14 @@ exports.duration = {
                 minutes: 9,
                 seconds: 20,
                 milliseconds: 12
-            });
+            }),
+            modified = moment.duration(1, 'day').add(moment.duration(1, 'day'));
 
-        test.expect(3);
+        test.expect(4);
         test.deepEqual(moment.duration(simple), simple, "simple clones are equal");
         test.deepEqual(moment.duration(lengthy), lengthy, "lengthy clones are equal");
         test.deepEqual(moment.duration(complicated), complicated, "complicated clones are equal");
+        test.deepEqual(moment.duration(modified), modified, "cloning modified duration works");
         test.done();
     },