警告：非数字值

Question

为什么会出现此错误：（我已升级到PHP 7 ...）

  

警告：在第1117行的/var/www/...../wp-content/plugins/wp-e-commerce/wpsc-includes/cart.class.php中遇到的非数字值

怎么了？

1116        foreach ( ( array ) $this->cart_items as $cart_item ) {

1117            $total += $cart_item->calculate_shipping( $method );

1118        }

1119        if ( $method == $this->selected_shipping_method ) {

1120            $this->total_item_shipping = $total;

1121        }

1122        return $total;