diff --git a/toc_api.module b/toc_api.module
index 62aa9c33b15789abe86c8d8beaf380bc8a86e49d..095434731b9e2fbe7540d027631e4e11023c996d 100755
--- a/toc_api.module
+++ b/toc_api.module
@@ -120,7 +120,7 @@ function template_preprocess_toc_tree(&$variables) {
   $variables['options'] = $toc->getOptions();
 
   // Add default classes.
-  $variables['attributes']['class'] = (array) $variables['attributes']['class'];
+  $variables['attributes']['class'] = isset($variables['attributes']['class']) ? (array) $variables['attributes']['class'] : [];
   $variables['attributes']['class'][] = 'toc';
   $variables['attributes']['class'][] = 'toc-tree';