diff -u b/core/lib/Drupal/Core/Site/Settings.php b/core/lib/Drupal/Core/Site/Settings.php
--- b/core/lib/Drupal/Core/Site/Settings.php
+++ b/core/lib/Drupal/Core/Site/Settings.php
@@ -46,10 +46,11 @@
    * @return \Drupal\Core\Site\Settings
    *
    * @throws \BadMethodCallException
+   *  Thrown when the settings instance called before being initialized.
    */
   public static function getInstance() {
     if (self::$instance === NULL) {
-      throw new \BadMethodCallException('Settings::$instance is not initialized yet.');
+      throw new \BadMethodCallException('Settings::$instance is not initialized yet. Call Settings::initialize() first.');
     }
     return self::$instance;
   }
diff -u b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
--- b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
+++ b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
@@ -139,7 +139,7 @@
     $instace_property->setAccessible(TRUE);
     $instace_property->setValue(NULL);
 
-    $this->setExpectedException('BadMethodCallException');
+    $this->setExpectedException(\BadMethodCallException::class);
     $settings->getInstance();
   }