diff -u b/core/modules/migrate_drupal/migrate_drupal.module b/core/modules/migrate_drupal/migrate_drupal.module
--- b/core/modules/migrate_drupal/migrate_drupal.module
+++ b/core/modules/migrate_drupal/migrate_drupal.module
@@ -46,7 +46,8 @@
       ],
     ];
     $vocabulary_migration = \Drupal::service('plugin.manager.migration')->createStubMigration($vocabulary_migration_definition);
-    $translation_active = \Drupal::service('module_handler')->moduleExists('config_translation');
+    $module_handler = \Drupal::service('module_handler');
+    $translation_active = $module_handler->moduleExists('config_translation') && $module_handler->moduleExists('content_translation');
 
     try {
       $source_plugin = $vocabulary_migration->getSourcePlugin();
@@ -66,17 +67,19 @@
           $plugin_ids[] = 'd6_term_node_translation:' . $source_vid;
         }
         foreach ($plugin_ids as $plugin_id) {
-          // Match the field name derivation in d6_vocabulary_field.yml.
-          $field_name = substr('field_' . $row->getDestinationProperty('vid'), 0, 32);
+          if (isset($definitions[$plugin_id])) {
+            // Match the field name derivation in d6_vocabulary_field.yml.
+            $field_name = substr('field_' . $row->getDestinationProperty('vid'), 0, 32);
 
-          // The Forum module is expecting 'taxonomy_forums' as the field name
-          // for the forum nodes. The 'forum_vocabulary' source property is
-          // evaluated in Drupal\taxonomy\Plugin\migrate\source\d6\Vocabulary
-          // and is set to true if the vocabulary vid being migrated is the
-          // same as the one in the 'forum_nav_vocabulary' variable on the
-          // source site.
-          $destination_vid = $row->getSourceProperty('forum_vocabulary') ? 'taxonomy_forums' : $field_name;
-          $definitions[$plugin_id]['process'][$destination_vid] = 'tid';
+            // The Forum module is expecting 'taxonomy_forums' as the field name
+            // for the forum nodes. The 'forum_vocabulary' source property is
+            // evaluated in Drupal\taxonomy\Plugin\migrate\source\d6\Vocabulary
+            // and is set to true if the vocabulary vid being migrated is the
+            // same as the one in the 'forum_nav_vocabulary' variable on the
+            // source site.
+            $destination_vid = $row->getSourceProperty('forum_vocabulary') ? 'taxonomy_forums' : $field_name;
+            $definitions[$plugin_id]['process'][$destination_vid] = 'tid';
+          }
         }
       }
     }
@@ -113,28 +116,28 @@
 
     if (isset($definitions['d6_comment'])) {
       $tmp = $definitions['d6_comment']['process']['entity_id'][0]['migration'];
-      $definitions['d6_comment']['process']['entity_id'][0]['migration'] = _insert_migration($tmp);;
+      $definitions['d6_comment']['process']['entity_id'][0]['migration'] = _insert_migration($tmp);
     }
     if (isset($definitions['d6_term_node'])) {
       $tmp = $definitions['d6_term_node']['process']['nid'][0]['migration'];
-      $definitions['d6_term_node']['process']['nid'][0]['migration'] = _insert_migration($tmp);;
+      $definitions['d6_term_node']['process']['nid'][0]['migration'] = _insert_migration($tmp);
     }
     if (isset($definitions['d6_term_node_revision'])) {
       $tmp = $definitions['d6_term_node_revision']['process']['vid'][0]['migration'];
-      $definitions['d6_term_node_revision']['process']['vid'][0]['migration'] = _insert_migration($tmp);;
+      $definitions['d6_term_node_revision']['process']['vid'][0]['migration'] = _insert_migration($tmp);
     }
     if (isset($definitions['d6_term_node_translation'])) {
       $tmp = $definitions['d6_term_node_translation']['process']['dest_nid'][0]['migration'];
-      $definitions['d6_term_node_translation']['process']['dest_nid'][0]['migration'] = _insert_migration($tmp);;
+      $definitions['d6_term_node_translation']['process']['dest_nid'][0]['migration'] = _insert_migration($tmp);
     }
     if (isset($definitions['d6_url_alias'])) {
       $tmp = $definitions['d6_url_alias']['process']['node_translation'][2]['migration'];
-      $definitions['d6_url_alias']['process']['node_translation'][2]['migration'] = _insert_migration($tmp);;
+      $definitions['d6_url_alias']['process']['node_translation'][2]['migration'] = _insert_migration($tmp);
     }
 
     if (isset($definitions['d7_comment'])) {
       $tmp = $definitions['d7_comment']['process']['entity_id'][0]['migration'];
-      $definitions['d7_comment']['process']['entity_id'][0]['migration'] = _insert_migration($tmp);;
+      $definitions['d7_comment']['process']['entity_id'][0]['migration'] = _insert_migration($tmp);
     }
     if (isset($definitions['d7_url_alias'])) {
       $tmp = $definitions['d7_url_alias']['process']['node_translation'][2]['migration'];