diff --git a/modules/system/system.module b/modules/system/system.module
index 4bed9c1..e317254 100644
--- a/modules/system/system.module
+++ b/modules/system/system.module
@@ -920,7 +920,14 @@ function _system_theme_data() {
     $themes_info = $themes;
   }
 
-  return $themes_info;
+  // To avoid side effects, we don't return the original objects
+  $themes_info_cloned = array();
+  // only PHP 5+ knows the "clone" keyword.
+  // PHP 4 clones objects by itself, unless they are by reference variables.
+  foreach ($themes_info as $key => $theme) {
+    $themes_info_cloned[$key] = drupal_clone($theme);
+  }
+  return $themes_info_cloned;
 }
 
 /**