diff --git a/include/functions_user.inc.php b/include/functions_user.inc.php
index 932b5d362..ab9633cb9 100644
--- a/include/functions_user.inc.php
+++ b/include/functions_user.inc.php
@@ -298,8 +298,10 @@ function build_user($user_id, $use_cache=true)
     $user['internal_status']['guest_must_be_guest'] = true;
   }
 
-  // Check user theme
-  if (!isset($user['theme_name']) || !check_theme_installed($user['theme_name']))
+  // Check user theme. 2 possible problems:
+  // 1. the user_infos.theme was not found in the themes table, thus themes.name is null
+  // 2. the theme is not really installed on the filesystem
+  if (!isset($user['theme_name']) or !check_theme_installed($user['theme']))
   {
     $user['theme'] = get_default_theme();
     $user['theme_name'] = $user['theme'];