define $transients_to_clear

includes/wc-order-functions.php

@@ -523,6 +523,7 @@ function wc_processing_order_count() {
  */
 function wc_delete_shop_order_transients( $post_id = 0 ) {
 	$post_id             = absint( $post_id );
+	$transients_to_clear = array();
 
 	// Clear report transients
 	$reports = WC_Admin_Reports::get_reports();