diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 089cc59ef1..a5b3ed4908 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -53,7 +53,6 @@ $multisite = $multisite || ( defined( 'MULTISITE' ) && MULTISITE );
 require_once( dirname( __FILE__ ) . '/mock-mailer.php' );
 $phpmailer = new MockPHPMailer();
 
-// Set the theme to our special empty theme, to avoid interference from the current Twenty* theme.
 if ( ! defined( 'WP_DEFAULT_THEME' ) ) {
 	define( 'WP_DEFAULT_THEME', 'default' );
 }
diff --git a/tests/phpunit/includes/install.php b/tests/phpunit/includes/install.php
index 50efd43bd7..38db3ce55a 100644
--- a/tests/phpunit/includes/install.php
+++ b/tests/phpunit/includes/install.php
@@ -9,15 +9,14 @@ error_reporting( E_ALL & ~E_DEPRECATED & ~E_STRICT );
 $config_file_path = $argv[1];
 $multisite = ! empty( $argv[2] );
 
+define( 'WP_INSTALLING', true );
+require_once $config_file_path;
+require_once dirname( __FILE__ ) . '/functions.php';
+
 // Set the theme to our special empty theme, to avoid interference from the current Twenty* theme.
 if ( ! defined( 'WP_DEFAULT_THEME' ) ) {
 	define( 'WP_DEFAULT_THEME', 'default' );
 }
-$wp_theme_directories = array( dirname( __FILE__ ) . '/../data/themedir1' );
-
-define( 'WP_INSTALLING', true );
-require_once $config_file_path;
-require_once dirname( __FILE__ ) . '/functions.php';
 
 tests_reset__SERVER();
 
@@ -33,6 +32,8 @@ global $phpmailer;
 require_once( dirname( __FILE__ ) . '/mock-mailer.php' );
 $phpmailer = new MockPHPMailer();
 
+register_theme_directory( dirname( __FILE__ ) . '/../data/themedir1' );
+
 /*
  * default_storage_engine and storage_engine are the same option, but storage_engine
  * was deprecated in MySQL (and MariaDB) 5.5.3, and removed in 5.7.