why doesn't /?product_category=slug work for MarketPress product categories?

Suppose we have a product category with the slug 'bass-guitars-electric'.
The normal URL would be something like this: http://makrismusic.gr/store/products/category/bass-guitars-electric/

Sometimes though, we need to query products using more than that... So we need to use something like this: http://makrismusic.gr/?product_category=bass-guitars-electric
Unfortunately this returns a 500 error.

After looking at the apache error logs this is what I got:

PHP Catchable fatal error: Object of class WP_Error could not be converted to string in /...../...../wp-includes/pluggable.php on line 894

When using the exact same method with other taxonomies There are no errors. For example, on the same site I have created a custom taxonomy vocabulary called "brand". When going to http://makrismusic.gr/?brand=fender we get no errors.

This could be caused by the custom product categories slugs though I 'm not sure.
Any advice would be appreciated...
This issue is related to this one (This post is the actual problem, the one linked here is the desired outcome): https://premium.wpmudev.org/forums/topic/marketpress-custompress-taxonomy-queries