diff --git src/wp-includes/update.php src/wp-includes/update.php
index c73ee0a..87cc3ce 100644
--- src/wp-includes/update.php
+++ src/wp-includes/update.php
@@ -654,13 +654,20 @@ function wp_schedule_update_checks() {
  * @since 4.1.0
  */
 function wp_clean_update_cache() {
+	// In case `wp_clean_plugins_cache()` does not exist, clear the transient directly.
 	if ( function_exists( 'wp_clean_plugins_cache' ) ) {
 		wp_clean_plugins_cache();
 	} else {
 		delete_site_transient( 'update_plugins' );
 	}
-	wp_clean_plugins_cache();
-	wp_clean_themes_cache();
+
+	// In case `wp_clean_themes_cache()` does not exist, clear the transient directly.
+	if ( function_exists( 'wp_clean_themes_cache' ) ) {
+		wp_clean_themes_cache();
+	} else {
+		delete_site_transient( 'update_themes' );
+	}
+
 	delete_site_transient( 'update_core' );
 }