diff --git a/core/config/schema/core.data_types.schema.yml b/core/config/schema/core.data_types.schema.yml
index a3b6c62..827fe92 100644
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
@@ -1,3 +1,23 @@
+# Base types provided by Drupal core.
+
+# Read https://drupal.org/node/1905070 for more details about configuration
+# schema, types and type resolution.
+
+# Undefined type used by the system to assign to elements at any level where
+# configuration schema is not defined. Using explicitly has the same effect as
+# not defining schema, so there is no point in doing that.
+undefined:
+  label: 'Undefined'
+  class: '\Drupal\Core\Config\Schema\Undefined'
+
+# Explicit type to use when no data typing is possible. Instead of using this
+# type, we strongly suggest you use configuration structures that can be
+# described with other structural elements of schema, and describe your schema
+# with those elements.
+ignore:
+  label: 'Ignore'
+  class: '\Drupal\Core\Config\Schema\Ignore'
+
 # Basic scalar data types from typed data.
 boolean:
   label: 'Boolean'
@@ -18,23 +38,13 @@ uri:
   label: 'Uri'
   class: '\Drupal\Core\TypedData\Plugin\DataType\Uri'
 
-# Undefined type to use for missing schema, don't use explicitly.
-undefined:
-  label: 'Undefined'
-  class: '\Drupal\Core\Config\Schema\Undefined'
-
-# Explicit type when no data typing is possible. Avoid if at all possible.
-ignore:
-  label: 'Ignore'
-  class: '\Drupal\Core\Config\Schema\Ignore'
-
-# Container data types with lists with known and uknown keys.
-sequence:
-  label: Sequence
-  class: '\Drupal\Core\Config\Schema\Sequence'
+# Container data types for lists with known and unknown keys.
 mapping:
   label: Mapping
   class: '\Drupal\Core\Config\Schema\Mapping'
+sequence:
+  label: Sequence
+  class: '\Drupal\Core\Config\Schema\Sequence'
 
 # Simple extended data types:
 
@@ -61,6 +71,11 @@ date_format:
   label: 'PHP date format'
   translatable: true
 
+# HTML color value.
+color_hex:
+  type: string
+  label: 'Color'
+
 # Complex extended data types:
 
 # Mail text with subject and body parts.
@@ -196,11 +211,6 @@ route:
         - type: string
           label: 'Param'
 
-# HTML color value.
-color_hex:
-  type: string
-  label: 'Color'
-
 # Config dependencies.
 config_dependencies:
   type: mapping
diff --git a/core/lib/Drupal/Core/Config/StorableConfigBase.php b/core/lib/Drupal/Core/Config/StorableConfigBase.php
index e5b313d..9c0fba8 100644
--- a/core/lib/Drupal/Core/Config/StorableConfigBase.php
+++ b/core/lib/Drupal/Core/Config/StorableConfigBase.php
@@ -174,8 +174,10 @@ protected function castValue($key, $value) {
       $element = $this->getSchemaWrapper()->get($key);
     }
     catch (SchemaIncompleteException $e) {
-      // @todo throw an exception due to an incomplete schema.
-      // Fix as part of https://drupal.org/node/2183983.
+      // @todo Consider making schema handling more strict by throwing
+      // SchemaIncompleteException for all incomplete schema conditions *and*
+      // throwing it forward. See https://drupal.org/node/2183983.
+      // Until then, we need to handle the Undefined case below.
     }
     // Do not cast value if it is unknown or defined to be ignored.
     if ($element && ($element instanceof Undefined || $element instanceof Ignore)) {