Error message on checkout step 2 when using a coupon

[mc12345678]

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.