diff --git a/tests/phpunit/tests/theme.php b/tests/phpunit/tests/theme.php
index f6a418d3d3..c7742536a4 100644
--- a/tests/phpunit/tests/theme.php
+++ b/tests/phpunit/tests/theme.php
@@ -23,13 +23,20 @@ class Tests_Theme extends WP_UnitTestCase {
 		'twentytwentytwo',
 	);
 
+	/**
+	 * Original theme directory.
+	 *
+	 * @var string[]
+	 */
+	private $orig_theme_dir;
+
 	public function set_up() {
 		global $wp_theme_directories;
 
 		parent::set_up();
 
-		$backup_wp_theme_directories = $wp_theme_directories;
-		$wp_theme_directories        = array( WP_CONTENT_DIR . '/themes' );
+		$this->orig_theme_dir = $wp_theme_directories;
+		$wp_theme_directories = array( WP_CONTENT_DIR . '/themes' );
 
 		add_filter( 'extra_theme_headers', array( $this, 'theme_data_extra_headers' ) );
 		wp_clean_themes_cache();
@@ -39,7 +46,7 @@ class Tests_Theme extends WP_UnitTestCase {
 	public function tear_down() {
 		global $wp_theme_directories;
 
-		$wp_theme_directories = $this->wp_theme_directories;
+		$wp_theme_directories = $this->orig_theme_dir;
 
 		remove_filter( 'extra_theme_headers', array( $this, 'theme_data_extra_headers' ) );
 		wp_clean_themes_cache();