diff --git a/lodash.js b/lodash.js
index e92f31e89..2fc38ccf4 100644
--- a/lodash.js
+++ b/lodash.js
@@ -4786,10 +4786,10 @@
      */
     function mergeDefaults(objValue, srcValue, key, object, source, stack) {
       if (isObject(objValue)) {
-        stack.set(objValue, objValue);
+        stack.set(srcValue, objValue);
         baseMerge(objValue, srcValue, mergeDefaults, stack);
       }
-      return objValue === undefined ? srcValue : objValue;
+      return objValue === undefined ? baseClone(srcValue) : objValue;
     }
 
     /**
diff --git a/test/test.js b/test/test.js
index b96a67f1d..2f14c3d79 100644
--- a/test/test.js
+++ b/test/test.js
@@ -3562,7 +3562,20 @@
       source.bar.b = source.foo.b;
 
       var actual = _.defaultsDeep(object, source);
-      assert.ok(actual.bar.b === source.foo.b && actual.foo.b.c.d === actual.foo.b.c.d.foo.b.c.d);
+      assert.ok(actual.bar.b === actual.foo.b && actual.foo.b.c.d === actual.foo.b.c.d.foo.b.c.d);
+    });
+
+    QUnit.test('should not modify sources', function(assert) {
+      assert.expect(3);
+
+      var object = {},
+          source1 = { 'a': 1, 'b': { 'c': 2 } },
+          source2 = { 'b': { 'c': 3, 'd': 3 } },
+          actual = _.defaultsDeep(object, source1, source2);
+
+      assert.deepEqual(actual, { 'a': 1, 'b': { 'c': 2, 'd': 3 } });
+      assert.deepEqual(source1, { 'a': 1, 'b': { 'c': 2 } });
+      assert.deepEqual(source2, { 'b': { 'c': 3, 'd': 3 } });
     });
   }());