diff --git a/devel_themer.module b/devel_themer.module
index 11e8929..bb0fbc4 100644
--- a/devel_themer.module
+++ b/devel_themer.module
@@ -198,7 +198,7 @@ function devel_themer_shutdown() {
  *
  * @return void
  **/
-function devel_themer_theme_registry_alter($theme_registry) {
+function devel_themer_theme_registry_alter(&$theme_registry) {
   foreach ($theme_registry as $hook => $data) {
     if (isset($theme_registry[$hook]['function'])) {
       // If the hook is a function, store it so it can be run after it has been intercepted.