diff --git a/core/config/schema/core.data_types.schema.yml b/core/config/schema/core.data_types.schema.yml
index 0fbeeed..a0bd01a 100644
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
@@ -46,7 +46,7 @@ mapping:
 sequence:
   label: Sequence
   class: '\Drupal\Core\Config\Schema\Sequence'
-  definition_class: '\Drupal\Core\TypedData\ListDataDefinition'
+  definition_class: '\Drupal\Core\Config\Schema\SequenceDataDefinition'
 
 # Simple extended data types:
 
diff --git a/core/lib/Drupal/Core/Config/Schema/Sequence.php b/core/lib/Drupal/Core/Config/Schema/Sequence.php
index 7b98761..26dc1b8 100644
--- a/core/lib/Drupal/Core/Config/Schema/Sequence.php
+++ b/core/lib/Drupal/Core/Config/Schema/Sequence.php
@@ -2,8 +2,6 @@
 
 namespace Drupal\Core\Config\Schema;
 
-use Drupal\Core\TypedData\TypedDataInterface;
-
 /**
  * Defines a configuration element of type Sequence.
  *
@@ -18,17 +16,6 @@ class Sequence extends ArrayElement {
   /**
    * {@inheritdoc}
    */
-  public function __construct($definition, $name = NULL, TypedDataInterface $parent = NULL) {
-    parent::__construct($definition, $name, $parent);
-
-    if (!isset($this->definition['orderby'])) {
-      $this->definition['orderby'] = 'none';
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   protected function getElementDefinition($key) {
     $value = isset($this->value[$key]) ? $this->value[$key] : NULL;
     // @todo: Remove BC layer for sequence with hyphen in front. https://www.drupal.org/node/2444979
@@ -42,4 +29,20 @@ protected function getElementDefinition($key) {
     return $this->buildDataDefinition($definition, $value, $key);
   }
 
+  /**
+   * Gets the description of how the sequence should be sorted.
+   *
+   * @return string|null
+   *   Describes how the sequence should be sorted. Either 'value', 'key' or
+   *   NULL. NULL means that the schema does not describe how the sequence
+   *   should be sorted.
+   */
+  public function getOrderBy() {
+    $definition = $this->getDataDefinition();
+    if ($definition instanceof SequenceDataDefinition) {
+      return $definition->getOrderBy();
+    }
+    return NULL;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Config/Schema/SequenceDataDefinition.php b/core/lib/Drupal/Core/Config/Schema/SequenceDataDefinition.php
new file mode 100644
index 0000000..6264825
--- /dev/null
+++ b/core/lib/Drupal/Core/Config/Schema/SequenceDataDefinition.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\Core\Config\Schema;
+
+use Drupal\Core\TypedData\ListDataDefinition;
+
+/**
+ * A typed data definition class for defining sequences in configuration.
+ */
+class SequenceDataDefinition extends ListDataDefinition {
+
+  /**
+   * Gets the description of how the sequence should be sorted.
+   *
+   * @return string|null
+   *   Describes how the sequence should be sorted. Either 'value', 'key' or
+   *   NULL. NULL means that the schema does not describe how the sequence
+   *   should be sorted.
+   */
+  public function getOrderBy() {
+    return isset($this->definition['orderby']) ? $this->definition['orderby'] : NULL;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Config/StorableConfigBase.php b/core/lib/Drupal/Core/Config/StorableConfigBase.php
index d0c89e9..33b9323 100644
--- a/core/lib/Drupal/Core/Config/StorableConfigBase.php
+++ b/core/lib/Drupal/Core/Config/StorableConfigBase.php
@@ -214,7 +214,7 @@ protected function castValue($key, $value) {
 
       if ($element instanceof Sequence) {
         // Apply any sorting defined on the schema.
-        switch ($element->getDataDefinition()['orderby']) {
+        switch ($element->getOrderBy()) {
           case 'key':
             ksort($value);
             break;