FreeAgent Payroll Expert needed to check payroll

Hi, I need help with my employees first payroll. I am using FreeAgent to run my payroll for my business. I have the employees P45 and have set them up on the payroll, but when its generating the payslip, the employees tax is showing as a negative number and I am not sure why. Would appreciate a FreeAgent / payroll expert to run their eyes over it as I really don't want to get it wrong for our employee and them end up underpaying tax and having a bill at some point in the future!