diff --git a/core/includes/module.inc b/core/includes/module.inc
index 1a32c09..65aa2d4 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -135,7 +135,10 @@ function system_list_reset() {
  */
 function system_register($type, $name, $uri) {
   drupal_get_filename($type, $name, $uri);
-  drupal_classloader_register($name, dirname($uri));
+  // Do not register namespaces for themes.
+  if ($type != 'theme') {
+    drupal_classloader_register($name, dirname($uri));
+  }
 }
 
 /**