diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 0bf5a97..a66d097 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -89,6 +89,14 @@
  * @see install_state_defaults()
  */
 function install_drupal($class_loader, $settings = array()) {
+  // Support the old way of calling this function with just a settings array.
+  // @todo Remove this when Drush is updated in the Drupal testing
+  //   infrastructure.
+  if (is_array($class_loader) && $settings === array()) {
+    $settings = $class_loader;
+    $class_loader = require_once __DIR__ . '../vendor/autoload.php';
+  }
+
   global $install_state;
   // Initialize the installation state with the settings that were passed in,
   // as well as a boolean indicating whether or not this is an interactive