When creating manual credit with amount equal zero Koha explodes with a message: "Exception 'Koha::Exceptions::Account::AmountNotPositive' thrown 'Debit amount passed is not positive' at /usr/share/perl5/Exception/Class/Base.pm line 88". To reproduce: 1. Go to the patron -> Accounting -> Create manual credit. 2. Enter 0 in amount field. Click 'Add credit' button. 3. Koha explodes.