diff -u b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Boolean.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Boolean.php
--- b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Boolean.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Boolean.php
@@ -30,5 +30,5 @@
    */
   public function getCastedValue() {
-    return ($this->value !== '' ? (bool) $this->value : NULL);
+    return (bool) $this->value;
   }
 }
diff -u b/core/lib/Drupal/Core/TypedData/PrimitiveInterface.php b/core/lib/Drupal/Core/TypedData/PrimitiveInterface.php
--- b/core/lib/Drupal/Core/TypedData/PrimitiveInterface.php
+++ b/core/lib/Drupal/Core/TypedData/PrimitiveInterface.php
@@ -29,7 +29,7 @@
   public function setValue($value);
 
   /**
-   * Gets the primitive data value casted to the correct type.
+   * Gets the primitive data value casted to the correct PHP type.
    *
    * @return mixed
    */
diff -u b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
--- b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigSchemaTest.php
@@ -266,6 +266,8 @@
       'null_float' => '',
       // Not in schema and therefore should be left untouched.
       'not_present_in_schema' => TRUE,
+      // Test a custom type.
+      'config_test_integer' => '1',
     );
     $untyped_to_typed = $untyped_values;
 
@@ -283,6 +285,7 @@
       'float' => 3.14,
       'null_float' => NULL,
       'not_present_in_schema' => TRUE,
+      'config_test_integer' => 1,
     );
 
     // Save config which has a schema that enforces types.
@@ -293,8 +296,8 @@
 
     // Save config which does not have a schema that enforces types.
     \Drupal::config('config_test.no_schema_data_types')
-           ->setData($untyped_values)
-           ->save();
+      ->setData($untyped_values)
+      ->save();
     $this->assertIdentical(\Drupal::config('config_test.no_schema_data_types')->get(), $untyped_values);
   }
 
diff -u b/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
--- b/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
@@ -92,10 +92,16 @@
       type: string
       label: 'Protected property'
 
+config_test_integer:
+  type: integer
+  label: 'Config test integer'
+
 config_test.schema_data_types:
   type: mapping
   label: 'Config test schema'
   mapping:
+    config_test_integer:
+      type: config_test_integer
     integer:
       type: integer
     null_integer: