diff -u b/src/Plugin/Field/FieldType/Gender.php b/src/Plugin/Field/FieldType/Gender.php
--- b/src/Plugin/Field/FieldType/Gender.php
+++ b/src/Plugin/Field/FieldType/Gender.php
@@ -2,11 +2,12 @@
 
 namespace Drupal\gender\Plugin\Field\FieldType;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\TypedData\DataDefinition;
-use Drupal\options\Plugin\Field\FieldType\ListStringItem;
+use Drupal\options\Plugin\Field\FieldType\ListItemBase;
 
 /**
  * Plugin implementation of the gender field type.
@@ -21,7 +22,7 @@
  *   default_formatter = "list_default"
  * )
  */
-class Gender extends ListStringItem {
+class Gender extends ListItemBase {
 
   /**
    * {@inheritdoc}
@@ -38,6 +39,23 @@
   /**
    * {@inheritdoc}
    */
+  public static function schema(FieldStorageDefinitionInterface $field_definition) {
+    return [
+      'columns' => [
+        'value' => [
+          'type' => 'varchar',
+          'length' => 255,
+        ],
+      ],
+      'indexes' => [
+        'value' => ['value'],
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
     return [];
   }
@@ -53,2 +71,25 @@
 
+  /**
+   * {@inheritdoc}
+   */
+  protected static function validateAllowedValue($option) {
+    if (Unicode::strlen($option) > 255) {
+      return t('Allowed values list: each key must be a string at most 255 characters long.');
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function castAllowedValue($value) {
+    return (string) $value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function allowedValuesDescription() {
+    return '';
+  }
+
 }