diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 0551e56..0db55aa 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1085,7 +1085,7 @@ function drupal_page_is_cacheable($allow_caching = NULL) {
  * @see bootstrap_hooks()
  */
 function bootstrap_invoke_all($hook) {
-  foreach (module_list('bootstrap') as $module) {
+  foreach (module_list(MODULE_LIST_BOOTSTRAP) as $module) {
     drupal_load('module', $module);
     module_invoke($module, $hook);
   }
diff --git a/core/includes/module.inc b/core/includes/module.inc
index a3de431..1fb7a7c 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -8,6 +8,16 @@
 use Drupal\Component\Graph\Graph;
 
 /**
+ * All enabled modules required for bootstrap.
+ */
+define(MODULE_LIST_BOOTSTRAP, 1);
+
+/**
+ * All enabled modules.
+ */
+define(MODULE_LIST_ENABLED, 2);
+
+/**
  * Load all the modules that have been enabled in the system table.
  *
  * @param $bootstrap
@@ -30,7 +40,7 @@ function module_load_all($bootstrap = FALSE) {
   $has_run = &$drupal_static_fast['has_run'];
 
   if (isset($bootstrap)) {
-    $type = $bootstrap ? 'bootstrap' : 'module_enabled';
+    $type = $bootstrap ? MODULE_LIST_BOOTSTRAP : MODULE_LIST_ENABLED;
     foreach (module_list($type) as $module) {
       drupal_load('module', $module);
     }
@@ -51,10 +61,10 @@ function module_load_all($bootstrap = FALSE) {
  * which case system_list() is omitted and the fixed list is always returned in
  * subsequent calls until manually reverted via module_list_reset().
  *
- * @param string $type
+ * @param int $type
  *   The type of list to return:
- *   - module_enabled: All enabled modules.
- *   - bootstrap: All enabled modules required for bootstrap.
+ *   - MODULE_LIST_ENABLED: All enabled modules.
+ *   - MODULE_LIST_BOOTSTRAP: All enabled modules required for bootstrap.
  * @param array $fixed_list
  *   (optional) An array of module names to override the list of modules. This
  *   list will persist until the next call with a new $fixed_list passed in.
@@ -82,7 +92,7 @@ function module_list($type = 'module_enabled', array $fixed_list = NULL) {
     $list = $module_list;
   }
   elseif (!isset($module_list)) {
-    if ($type == 'bootstrap') {
+    if ($type == MODULE_LIST_BOOTSTRAP) {
       $list = system_list('bootstrap');
     }
     else {