diff --git a/lodash.js b/lodash.js
index 0944c4c20..8ac3ce3c5 100644
--- a/lodash.js
+++ b/lodash.js
@@ -2462,7 +2462,7 @@
      */
     function assignMergeValue(object, key, value) {
       if ((value !== undefined && !eq(object[key], value)) ||
-          (typeof key == 'number' && value === undefined && !(key in object))) {
+          (value === undefined && !(key in object))) {
         baseAssignValue(object, key, value);
       }
     }
@@ -3616,29 +3616,32 @@
       var isCommon = newValue === undefined;
 
       if (isCommon) {
+        var isArr = isArray(srcValue),
+            isTyped = !isArr && isTypedArray(srcValue);
+
         newValue = srcValue;
-        if (isArray(srcValue) || isTypedArray(srcValue)) {
+        if (isArr || isTyped) {
           if (isArray(objValue)) {
             newValue = objValue;
           }
           else if (isArrayLikeObject(objValue)) {
             newValue = copyArray(objValue);
           }
-          else {
+          else if (isTyped) {
             isCommon = false;
-            newValue = baseClone(srcValue, true);
+            newValue = cloneTypedArray(srcValue, true);
+          }
+          else {
+            newValue = [];
           }
         }
         else if (isPlainObject(srcValue) || isArguments(srcValue)) {
+          newValue = objValue;
           if (isArguments(objValue)) {
             newValue = toPlainObject(objValue);
           }
           else if (!isObject(objValue) || (srcIndex && isFunction(objValue))) {
-            isCommon = false;
-            newValue = baseClone(srcValue, true);
-          }
-          else {
-            newValue = objValue;
+            newValue = initCloneObject(srcValue);
           }
         }
         else {
diff --git a/test/test.js b/test/test.js
index 586b0015b..c6759e487 100644
--- a/test/test.js
+++ b/test/test.js
@@ -4512,15 +4512,20 @@
     QUnit.test('should assign source properties if missing on `object`', function(assert) {
       assert.expect(1);
 
-      assert.deepEqual(_.defaults({ 'a': 1 }, { 'a': 2, 'b': 2 }), { 'a': 1, 'b': 2 });
+      var actual = _.defaults({ 'a': 1 }, { 'a': 2, 'b': 2 });
+      assert.deepEqual(actual, { 'a': 1, 'b': 2 });
     });
 
     QUnit.test('should accept multiple sources', function(assert) {
       assert.expect(2);
 
-      var expected = { 'a': 1, 'b': 2, 'c': 3 };
-      assert.deepEqual(_.defaults({ 'a': 1, 'b': 2 }, { 'b': 3 }, { 'c': 3 }), expected);
-      assert.deepEqual(_.defaults({ 'a': 1, 'b': 2 }, { 'b': 3, 'c': 3 }, { 'c': 2 }), expected);
+      var expected = { 'a': 1, 'b': 2, 'c': 3 },
+          actual = _.defaults({ 'a': 1, 'b': 2 }, { 'b': 3 }, { 'c': 3 });
+
+      assert.deepEqual(actual, expected);
+
+      actual = _.defaults({ 'a': 1, 'b': 2 }, { 'b': 3, 'c': 3 }, { 'c': 2 });
+      assert.deepEqual(actual, expected);
     });
 
     QUnit.test('should not overwrite `null` values', function(assert) {
@@ -4537,6 +4542,15 @@
       assert.strictEqual(actual.a, 1);
     });
 
+    QUnit.test('should assign `undefined` values', function(assert) {
+      assert.expect(1);
+
+      var source = { 'a': undefined, 'b': 1 },
+          actual = _.defaults({}, source);
+
+      assert.deepEqual(actual, { 'a': undefined, 'b': 1 });
+    });
+
     QUnit.test('should assign properties that shadow those on `Object.prototype`', function(assert) {
       assert.expect(2);
 
@@ -4560,8 +4574,11 @@
         'valueOf': 7
       };
 
-      assert.deepEqual(_.defaults({}, source), source);
-      assert.deepEqual(_.defaults({}, object, source), object);
+      var expected = lodashStable.clone(source);
+      assert.deepEqual(_.defaults({}, source), expected);
+
+      expected = lodashStable.clone(object);
+      assert.deepEqual(_.defaults({}, object, source), expected);
     });
   }());
 
@@ -4633,6 +4650,16 @@
       assert.strictEqual(actual.a.b, 2);
     });
 
+    QUnit.test('should assign `undefined` values', function(assert) {
+      assert.expect(1);
+
+      var source = { 'a': undefined, 'b': { 'c': undefined, 'd': 1 } },
+          expected = lodashStable.cloneDeep(source),
+          actual = _.defaultsDeep({}, source);
+
+      assert.deepEqual(actual, expected);
+    });
+
     QUnit.test('should merge sources containing circular references', function(assert) {
       assert.expect(2);
 
@@ -4672,7 +4699,7 @@
       assert.expect(1);
 
       var actual = _.defaultsDeep({ 'a': ['abc'] }, { 'a': 'abc' });
-      assert.deepEqual(actual, { 'a': ['abc'] });
+      assert.deepEqual(actual.a, ['abc']);
     });
   }());
 
@@ -14993,14 +15020,14 @@
       assert.deepEqual(actual, [new Foo(object)]);
     });
 
-    QUnit.test('should not assign `undefined` values', function(assert) {
+    QUnit.test('should not overwrite existing values with `undefined` values of object sources', function(assert) {
       assert.expect(1);
 
       var actual = _.merge({ 'a': 1 }, { 'a': undefined, 'b': undefined });
-      assert.deepEqual(actual, { 'a': 1 });
+      assert.deepEqual(actual, { 'a': 1, 'b': undefined });
     });
 
-    QUnit.test('should skip `undefined` values in array sources if a destination value exists', function(assert) {
+    QUnit.test('should not overwrite existing values with `undefined` values of array sources', function(assert) {
       assert.expect(2);
 
       var array = [1];