diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 8519961..63c22d0 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -494,6 +494,10 @@ function install_begin_request(&$install_state) {
     // Override the module list with a minimal set of modules.
     $module_handler->setModuleList(array('system' => 'core/modules/system/system.module'));
   }
+  // After setting up a custom and finite module list in a custom low-level
+  // bootstrap like here, ensure to use ModuleHandler::loadAll() so that
+  // ModuleHandler::isLoaded() returns TRUE, since that is a condition being
+  // checked by other subsystems (e.g., the theme system).
   $module_handler->loadAll();
 
   // Add list of all available profiles to the installation state.
diff --git a/core/includes/module.inc b/core/includes/module.inc
index e7cad81..dce73de 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -290,8 +290,9 @@ function drupal_required_modules() {
   $files = $listing->scan('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.module$/', 'modules');
   $required = array();
 
-  // An installation profile is required and one must always be loaded.
-  // The only exception is the installer.
+  // Unless called by the installer, an installation profile is required and
+  // must always be loaded. drupal_get_profile() also returns the installation
+  // profile in the installer, but only after it has been selected.
   if ($profile = drupal_get_profile()) {
     $required[] = $profile;
   }