Discount label error

Hi,
I have a problem with the discount label. It appears correctly but behind it there is also this error message:
Warning: A non-numeric value encountered in /usr/home/monaur.com/web/wp-content/themes/monaur/inc/woocommerce/woostify-woocommerce-template-functions.php on line 651

Any ideas of what could cause this issue?

Thanks in advance.

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