diff --git a/core/modules/forum/src/Controller/ForumController.php b/core/modules/forum/src/Controller/ForumController.php
index 6c1a308..aeb172c 100644
--- a/core/modules/forum/src/Controller/ForumController.php
+++ b/core/modules/forum/src/Controller/ForumController.php
@@ -232,8 +232,10 @@ protected function build($forums, TermInterface $term, $topics = array(), $paren
     foreach ($forums as $forum) {
       $this->renderer->addCacheableDependency($build, $forum);
     }
-    foreach ($topics as $topic) {
-      $this->renderer->addCacheableDependency($build, $topic);
+    if (!empty($topics)) {
+      foreach ($topics as $topic) {
+        $this->renderer->addCacheableDependency($build, $topic);
+      }
     }
     foreach ($parents as $parent) {
       $this->renderer->addCacheableDependency($build, $parent);