diff --git a/l10n_update.drush.inc b/l10n_update.drush.inc
index 087b60b..6ffb0f0 100644
--- a/l10n_update.drush.inc
+++ b/l10n_update.drush.inc
@@ -118,6 +118,21 @@ function drush_l10n_update() {
 
       // Batch update all projects for selected languages.
       $mode = drush_get_option('mode', 'keep');
+
+      switch ($mode) {
+        case 'keep':
+          $mode = LOCALE_IMPORT_KEEP;
+          break;
+
+        case 'overwrite':
+          $mode = LOCALE_IMPORT_OVERWRITE;
+          break;
+
+        default:
+          return drush_set_error('L10N_UPDATE_INVALID_MODE', dt('Invalid update mode. Valid options are keep, overwrite.'));
+          break;
+      }
+
       $languages = drush_get_option('languages');
       module_load_include('batch.inc', 'l10n_update');
       $updates = _l10n_update_prepare_updates($updates, NULL, array_keys($languages));