diff --git a/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
index b2fdb0d..f9febad 100644
--- a/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -570,7 +570,7 @@ protected function castValue($key, $value) {
         case 'Drupal\Core\TypedData\Plugin\DataType\Uri':
         case 'Drupal\Core\TypedData\Plugin\DataType\Email':
         case 'Drupal\Core\Config\Schema\Property':
-          $value = (string) $value;
+          $value = !isset($value) ? NULL : (string) $value;
           break;
         case 'Drupal\Core\TypedData\Plugin\DataType\Integer':
           if (!isset($value) || $value === '') {
diff --git a/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
index 8f956c9..c9bf579 100644
--- a/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
@@ -252,6 +252,7 @@ function testSchemaData() {
   public function testConfigSaveWithSchema() {
     $untyped_values = array(
       'string' => 1,
+      'null_string' => '',
       'integer' => '100',
       'null_integer' => '',
       'boolean' => 1,
@@ -265,9 +266,12 @@ public function testConfigSaveWithSchema() {
       // Not in schema and therefore should be left untouched.
       'not_present_in_schema' => TRUE,
     );
+    $untyped_to_typed = $untyped_values;
+    $untyped_to_typed['null_string'] = NULL;
 
     $typed_values = array(
       'string' => '1',
+      'null_string' => NULL,
       'integer' => 100,
       'null_integer' => NULL,
       'boolean' => TRUE,
@@ -282,7 +286,7 @@ public function testConfigSaveWithSchema() {
 
     // Save config which has a schema that enforces types.
     \Drupal::config('config_test.schema_data_types')
-      ->setData($untyped_values)
+      ->setData($untyped_to_typed)
       ->save();
     $this->assertIdentical(\Drupal::config('config_test.schema_data_types')->get(), $typed_values);
 
diff --git a/core/modules/config/tests/config_test/config/schema/config_test.schema.yml b/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
index c13216d..e0377bf 100644
--- a/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
+++ b/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
@@ -106,6 +106,8 @@ config_test.schema_data_types:
       type: float
     string:
       type: string
+    null_string:
+      type: string
     boolean:
       type: boolean
     no_type: