"Warning: A non-numeric value encountered in" in php 7.1.7

This kind of warning pops-up in php 7.1.7 on a cart page, when product is added to cart.

Warning: A non-numeric value encountered in /home/..../public_html/perch/addons/apps/perch_shop/lib/PerchShop_Product.class.php on line 409

it does not happen when I change php to 5.6.29

Perch: 3.0.8, PHP: 7.1.7-dh1, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 76b08b24596e12d4553bd41fc93cccd5bac2fe7a $, with PDO
Server OS: Linux, litespeed
Installed apps: content (3.0.8), assets (3.0.8), categories (3.0.8), perch_blog (5.5.1), perch_forms (1.9), perch_shop_orders (1.2.2), perch_shop_products (1.2.2), perch_shop (1.2.2), perch_members (1.6.2)
App runtimes: <?php $apps_list = [ 'perch_forms', 'perch_members', 'perch_shop', ];
PERCH_LOGINPATH: /perch
PERCH_PATH: /.../public_html/perch
PERCH_CORE: /.../public_html/perch/core
PERCH_RESFILEPATH: /.../public_html/perch/resources
Image manipulation: GD Imagick
PHP limits: Max upload 200M, Max POST 32M, Memory: 2048M, Total max file upload: 32M
F1: 0c66c2e1f82f9e0b7617b2cb8270f2c7
Resource folder writeable: Yes
HTTP_HOST: xxx
DOCUMENT_ROOT: /.../public_html
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php

debug

Array
(
    [type] => 2
    [message] => A non-numeric value encountered
    [file] => /.../perch/addons/apps/perch_shop/lib/PerchShop_Product.class.php
    [line] => 434
)