Filtering products with attribute value = 0 (zero)

Hi,
I am having an issue when I try to filter products with an attribute value = 0 (zero).
Seems that it evaluates the zero value as to be no filter, and reteieves all the products, not just the one with a value of zero.

See more details below.
Thanks in advance.

1. Go to https://avamoplast.intestmode.be/en/products/?swoof=1&pa_volume=0, this URL means that the filter attribute “volume” is set to zero.
2. It should retrieve the products with volume = zero, but instead retrieves all the products.

The page I need help with: [log in to see the link]