diff --git a/lodash.js b/lodash.js
index 05ecebe65..ae6941c22 100644
--- a/lodash.js
+++ b/lodash.js
@@ -3219,7 +3219,7 @@
           }
           else {
             isCommon = false;
-            newValue = baseClone(srcValue, !customizer);
+            newValue = baseClone(srcValue, true);
           }
         }
         else if (isPlainObject(srcValue) || isArguments(srcValue)) {
@@ -3228,7 +3228,7 @@
           }
           else if (!isObject(objValue) || (srcIndex && isFunction(objValue))) {
             isCommon = false;
-            newValue = baseClone(srcValue, !customizer);
+            newValue = baseClone(srcValue, true);
           }
           else {
             newValue = objValue;
diff --git a/test/test.js b/test/test.js
index f944db41c..776771810 100644
--- a/test/test.js
+++ b/test/test.js
@@ -14610,6 +14610,16 @@
       assert.deepEqual(actual, { 'a': { 'b': ['c'] } });
     });
 
+    QUnit.test('should clone sources when `customizer` result is `undefined`', function(assert) {
+      assert.expect(1);
+
+      var source1 = { 'a': { 'b': { 'c': 1 } } },
+          source2 = { 'a': { 'b': { 'd': 2 } } },
+          actual = _.mergeWith({}, source1, source2, alwaysUndefined);
+
+      assert.deepEqual(source1.a.b, { 'c': 1 });
+    });
+
     QUnit.test('should pop the stack of sources for each sibling property', function(assert) {
       assert.expect(1);