Ticket #31503: 31503.diff

src/wp-includes/update.php

@@ -654 +654 @@
  * @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' );
 }