diff --git a/src/wp-admin/includes/class-wp-upgrader.php b/src/wp-admin/includes/class-wp-upgrader.php
index 6c6cb5d828728..c85f96ef8457d 100644
--- a/src/wp-admin/includes/class-wp-upgrader.php
+++ b/src/wp-admin/includes/class-wp-upgrader.php
@@ -1008,8 +1008,18 @@ public function maintenance_mode( $enable = false ) {
 		global $wp_filesystem;
 
 		if ( ! $wp_filesystem ) {
-			require_once ABSPATH . 'wp-admin/includes/file.php';
-			WP_Filesystem();
+			if ( ! function_exists( 'WP_Filesystem' ) ) {
+				require_once ABSPATH . '/wp-admin/includes/file.php';
+			}
+
+			ob_start();
+			$credentials = request_filesystem_credentials( '' );
+			ob_end_clean();
+
+			if ( false === $credentials || ! WP_Filesystem( $credentials ) ) {
+				wp_trigger_error( __FUNCTION__, __( 'Could not access filesystem.' ) );
+				return;
+			}
 		}
 
 		$file = $wp_filesystem->abspath() . '.maintenance';