diff -u b/core/modules/system/system.install b/core/modules/system/system.install
--- b/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1227,14 +1227,34 @@
   $entity_installed_schema = \Drupal::keyValue('entity.storage_schema.sql');
   foreach (array_keys($entity_manager->getDefinitions()) as $entity_type_id) {
     if (($schema_data = $entity_installed_schema->get($entity_type_id . '.entity_schema_data')) && ($entity_type = $entity_manager->getLastInstalledDefinition($entity_type_id))) {
+      $id_field_name = $entity_type->getKey('id');
+      $default_langcode_field_name = $entity_type->getKey('default_langcode');
+      $langcode_field_name = $entity_type->getKey('langcode');
+
       $index_name = $entity_type_id . '__id__default_langcode__langcode';
-      $index_spec = [$entity_type->getKey('id'), $entity_type->getKey('default_langcode'), $entity_type->getKey('langcode')];
+      $index_fields = [$id_field_name, $default_langcode_field_name, $langcode_field_name];
+
+      // \Drupal\Core\Database\Schema::addIndex() requires some schema
+      // information about the fields being indexed in order to ensure that the
+      // index length isn't longer than the database allows. Here, we assume
+      // that for all entity types whose schema is managed by
+      // SqlContentEntityStorageSchema, that the field type plugin is:
+      // - 'integer': for the 'id' entity key field.
+      // - 'boolean': for the 'default_langcode' entity key field.
+      // - 'language': for the 'langcode' entity key field.
+      // The values below are what was returned by those field type plugins'
+      // schema() method at the time this update function was written.
+      $index_fields_spec = [
+        $id_field_name => ['type' => 'int', 'size' => 'normal'],
+        $default_langcode_field_name => ['type' => 'int', 'size' => 'tiny'],
+        $langcode_field_name => ['type' => 'varchar', 'length' => 12, 'is_ascii' => TRUE],
+      ];
 
       $new_schema_data = $schema_data;
       foreach ([$entity_type->getDataTable() ?: $entity_type_id . '_field_data', $entity_type->getRevisionDataTable() ?: $entity_type_id . '_field_revision'] as $table_name) {
         if (isset($schema_data[$table_name]) && !isset($schema_data[$table_name]['indexes'][$index_name]) && $db_schema->tableExists($table_name)) {
-          $db_schema->addIndex($table_name, $index_name, $index_spec);
-          $new_schema_data[$table_name]['indexes'][$index_name] = $index_spec;
+          $db_schema->addIndex($table_name, $index_name, $index_fields, ['fields' => $index_fields_spec]);
+          $new_schema_data[$table_name]['indexes'][$index_name] = $index_fields;
         }
       }
       if ($new_schema_data != $schema_data) {