Index: src/wp-admin/includes/class-wp-site-health.php
===================================================================
--- src/wp-admin/includes/class-wp-site-health.php	(revision 45343)
+++ src/wp-admin/includes/class-wp-site-health.php	(working copy)
@@ -612,7 +612,7 @@
 		}
 
 		// If not default Twenty* theme exists.
-		if ( ! $has_default_theme ) {
+		if ( ! $has_default_theme && class_exists( 'WP_Theme' ) && ! WP_Theme::get_core_default_theme() ) {
 			$result['status'] = 'recommended';
 
 			$result['label'] = __( 'Have a default theme available' );