diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php
index 5dc25d1..50f38ed 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php
@@ -27,21 +27,10 @@ class MapItem extends FieldItemBase {
    * {@inheritdoc}
    */
   public static function propertyDefinitions(FieldDefinitionInterface $field_definition) {
-    // There aren't any properties we can define statically.
-    return array();
-  }
+    $properties['value'] = DataDefinition::create('string')
+      ->setLabel(t('Serialized values'));
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getPropertyDefinitions() {
-    // Come up with some run-time property definitions, such that property
-    // objects can be used for all map values.
-    $definitions = parent::getPropertyDefinitions();
-    foreach ($this->values as $name => $value) {
-      $definitions[$name] = DataDefinition::create('any');
-    }
-    return $definitions;
+    return $properties;
   }
 
   /**