diff --git a/src/index.php b/src/index.php
index 0714b3beda..e5e130d105 100644
--- a/src/index.php
+++ b/src/index.php
@@ -11,13 +11,16 @@ if ( ! defined( 'ABSPATH' ) ) {
 	define( 'ABSPATH', __DIR__ . '/' );
 }
 
-define( 'WPINC', 'wp-includes' );
-
-if ( file_exists( ABSPATH . WPINC . '/js/dist/edit-post.js' ) ) {
+/*
+ * Load the actual index.php file if the assets were already built.
+ * Note: WPINC is not defined yet, it is defined later in wp-settings.php.
+ */
+if ( file_exists( ABSPATH . 'wp-includes/js/dist/edit-post.js' ) ) {
 	require_once ABSPATH . '_index.php';
 	return;
 }
 
+define( 'WPINC', 'wp-includes' );
 require_once ABSPATH . WPINC . '/load.php';
 
 // Standardize $_SERVER variables across setups.