Error message on checkout step 2 when using a coupon

I'm getting a fatal error trying to go from step 2 to 3 of checkout when using a coupon that has restrictions. If the coupon has no restrictions, checkout goes fine.

This is the debug message: Uncaught TypeError: validate_for_category(): Argument #1 ($product_id) must be of type int, string given, called in /includes/functions/functions_gvcoupons.php on line 182 and defined in /includes/functions/functions_gvcoupons.php:210

Don't know how to fix that or where to look for whatever's throwing the error. Help is appreciated. Thanks.

You've discovered a big in the software.
Change line 182 from:

    $allow_for_product = validate_for_product($product_id, $coupon_id);

To:

    $allow_for_product = validate_for_product((int)$product_id, $coupon_id);

Please report back the result which may include a similar statement about the coupon_id.

Then take credit for the discovery in the bus section of the forum.

This fixed it. No further errors came up, and the checkout process completed. Thank you!

Spoke too soon. The change also had to be made on line 183. Then there were no further errors. Thanks for your help.

I'll note that this currently occurs if-and-only-if a product with attributes (i.e. there's a string uprid as opposed to a simple products_id) is present in the cart.