Internal Server Error if payment_method.preferred_solution_type = "Mark"

[nxia416]

I am using 2-4-stable for every spree gem including better_spree_paypal_express. After better_spree_paypal_express implemented, there will be SOLUTION field with default value "Mark". However it will cause "Internal Server Error 500" from paypal. Same error for empty value. It seems it only works with "Sole".