diff --git a/install/index.php b/install/index.php
index 03c4f05a50ee..ca739c9606ec 100644
--- a/install/index.php
+++ b/install/index.php
@@ -1552,6 +1552,7 @@ public function install($config) {
 
         include_once "./core/includes/bootstrap.inc";
         include_once "./core/includes/unicode.inc";
+        include_once "./core/includes/config.inc";
 
         backdrop_bootstrap(BACKDROP_BOOTSTRAP_FULL);