I was testing something else and did not enter a discount amount, which resulted in an error 500. I think it should result in a warning or error message to tell the staff member that they forgot to enter a value in the discount input. To recreate: 1. Go to a patron account > Accounting 2. Click 'Create manual invoice' 3. Enter an amount 4. Click 'Save' 5. Click 'Apply discount' 6. Click 'Confirm' --> Error 500 It should say something like "Please enter a discount amount".