diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index cc67924..0b902a6 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -1723,9 +1723,11 @@ function hook_theme($existing, $type, $theme, $path) {
  */
 function hook_theme_registry_alter(&$theme_registry) {
   // Kill the next/previous forum topic navigation links.
-  foreach ($theme_registry['forum_topic_navigation']['preprocess functions'] as $key => $value) {
-    if ($value == 'template_preprocess_forum_topic_navigation') {
-      unset($theme_registry['forum_topic_navigation']['preprocess functions'][$key]);
+  if (!empty($theme_registry['forum_topic_navigation']['preprocess functions'])) {
+    foreach ($theme_registry['forum_topic_navigation']['preprocess functions'] as $key => $value) {
+      if ($value == 'template_preprocess_forum_topic_navigation') {
+        unset($theme_registry['forum_topic_navigation']['preprocess functions'][$key]);
+      }
     }
   }
 }