diff -u b/flag.module b/flag.module
--- b/flag.module
+++ b/flag.module
@@ -127,7 +127,12 @@
       continue;
     }
 
-    foreach ($flag->getFlaggableBundles() as $bundle_name) {
+    $flaggable_bundles = $flag->getFlaggableBundles();
+    if (empty($flaggable_bundles)) {
+      $flaggable_bundles = entity_get_bundles($flag->getFlaggableEntityType());
+    }
+
+    foreach ($flaggable_bundles as $bundle_name) {
       if ($flag_type_plugin->showOnForm()) {
         $extra[$flag->getFlaggableEntityType()][$bundle_name]['form']['flag'] = [
           'label' => t('Flags'),