diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 00fcec2..eb24109 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -664,7 +664,7 @@ public function addRebuildInfo($property, $value) {
   /**
    * {@inheritdoc}
    */
-  public function setStorage($storage) {
+  public function setStorage(array $storage) {
     $this->storage = $storage;
     return $this;
   }
@@ -672,7 +672,7 @@ public function setStorage($storage) {
   /**
    * {@inheritdoc}
    */
-  public function getStorage() {
+  public function &getStorage() {
     return $this->storage;
   }
 
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index 626ce34..f58322e 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -164,6 +164,24 @@ public function setRedirectUrl(Url $url);
   public function getRedirect();
 
   /**
+   * Sets the entire set of arbitrary data.
+   *
+   * @param array $storage
+   *   The entire set of arbitrary data to store for this form.
+   *
+   * @return $this
+   */
+  public function setStorage(array $storage);
+
+  /**
+   * Returns the entire set of arbitrary data.
+   *
+   * @return array
+   *   The entire set of arbitrary data to store for this form.
+   */
+  public function &getStorage();
+
+  /**
    * Gets any arbitrary property.
    *
    * @param string|array $property