diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 3f2f591..4c7223e 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1452,8 +1452,24 @@ function t($string, array $args = array(), array $options = array()) {
   static $custom_strings;
   $locale_exists = &drupal_static(__FUNCTION__);
 
+  // Check whether locale() is available and operational.
   if (!isset($locale_exits)) {
-    $locale_exists = function_exists('module_exists') && module_exists('locale');
+    // The availability of locale() requires more explanation:
+    // - Whether the module system has been loaded.
+    //   In early bootstrap, in the installer, update.php, and also in early
+    //   error/exception conditions, the module system might not be loaded yet.
+    // - Whether Locale module is enabled.
+    //   The mere existence of locale() does not imply that Locale module is
+    //   actually enabled and its database tables are installed. Since PHP code
+    //   cannot be unloaded, this is typically the case in the environment that
+    //   is executing a test.
+    // - Whether Locale module actually has been loaded.
+    //   The Locale module may be enabled, but the actual .module might not be
+    //   loaded. This is typically the case in update.php, which needs to
+    //   populate the full module list to invoke hook_requirements(), but must
+    //   not load any actual modules and APIs, since they are not guaranteed to
+    //   be operational.
+    $locale_exists = function_exists('module_exists') && module_exists('locale') && function_exists('locale');
   }
 
   // Merge in default.