diff --git a/core/modules/migrate_drupal/src/Plugin/MigrateCckFieldPluginManager.php b/core/modules/migrate_drupal/src/Plugin/MigrateCckFieldPluginManager.php
index 5f635d9..63a5030 100644
--- a/core/modules/migrate_drupal/src/Plugin/MigrateCckFieldPluginManager.php
+++ b/core/modules/migrate_drupal/src/Plugin/MigrateCckFieldPluginManager.php
@@ -18,12 +18,21 @@
 class MigrateCckFieldPluginManager extends MigratePluginManager {
 
   /**
+   * The default version of core to use for cck field plugins.
+   *
+   * These plugins were initially only built and used for Drupal 6 fields.
+   * Having been extended for Drupal 7 with a "core" annotation, we fall back to
+   * Drupal 6 where none exists.
+   */
+  const DEFAULT_CORE_VERSION = 6;
+
+  /**
    * {@inheritdoc}
    *
    * A specific createInstance method is necessary to pass the migration on.
    */
   public function createInstance($field_type, array $configuration = array(), MigrationInterface $migration = NULL) {
-    $core = 6;
+    $core = static::DEFAULT_CORE_VERSION;
     if (!empty($configuration['core'])) {
       $core = $configuration['core'];
     }
@@ -34,6 +43,7 @@ public function createInstance($field_type, array $configuration = array(), Migr
         }
       }
     }
+
     foreach ($this->getDefinitions() as $plugin_id => $definition) {
       if (in_array($core, $definition['core'])) {
         if (array_key_exists($field_type, $definition['type_map']) || $field_type === $plugin_id) {