"Input string was not in a correct format" custom action error

 [Input("Application Reference")]
        [ReferenceTarget("recoserv_application")]
        [Default("8B446743-C6C4-E711-A81C-005056B5FEFA", "recoserv_application")]
        public InArgument<EntityReference> ApplicationRef { get; set; }

        [Input("Amount")]
        public InArgument<string> Amount { get; set; }

        [Input("AuthCode")]
        public InArgument<string> Authcode { get; set; }

        [Output("PaymentID")]
        [AttributeTarget("recoserv_payment)", "recoserv_paymentid")]
        public OutArgument<string> PaymentId { get; set; }

        public override void ExecuteCRMWorkFlowActivity(CodeActivityContext executionContext, LocalWorkflowContext crmWorkflowContext)
        {
            //Create the tracing service
            ITracingService tracingService = executionContext.GetExtension<ITracingService>();
            //Create the context
            IWorkflowContext context = executionContext.GetExtension<IWorkflowContext>();
            IOrganizationServiceFactory serviceFactory = executionContext.GetExtension<IOrganizationServiceFactory>();
            IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);
            tracingService.Trace("applicationref: {0}", ApplicationRef.ToString());
            tracingService.Trace("amount: {0}", Amount);
            tracingService.Trace("authcode: {0}", Authcode);



            if (crmWorkflowContext == null)
            {
                throw new ArgumentNullException("crmWorkflowContext is null");
            }

	        try
	        {
                using (CrmServiceContext csc = new CrmServiceContext(service))
                {
                    var item = (from c in csc.recoserv_paymentSet
                                where c.recoserv_application.Id == this.ApplicationRef.Get(executionContext).Id
                                orderby c.CreatedOn descending
                                select c).FirstOrDefault();
                    //Entity payment = service.Retrieve("recoserv_payment", item.Id, new ColumnSet(true));
                    if (item!=null && item.StatusCode.Value ==1 && item.recoserv_actualamount.Value == decimal.Parse(Amount.ToString()))
                        {
                        tracingService.Trace("statuscode: {0}", item.StatusCode.Value.ToString());
                        tracingService.Trace("Payment ID: {0}", this.ApplicationRef.Get(executionContext).Id.ToString());
                        //update payment status
                        SetStateRequest request = new SetStateRequest();
                        request.EntityMoniker = new EntityReference(recoserv_payment.EntityLogicalName, item.Id);
                        request.State = new OptionSetValue((int)recoserv_paymentState.Active);
                        request.Status = new OptionSetValue((int)recoserv_payment_StatusCode.Paid); // Paid
                        SetStateResponse response = (SetStateResponse)service.Execute(request);
                        //update payment authcode
                        item.recoserv_authcode = Authcode.ToString();
                        //payment["recoserv_authcode"] = Authcode;
                        service.Update(item);
                        this.PaymentId.Set(executionContext, item.recoserv_paymentId);
                    }
                }
            }
            catch (FaultException<OrganizationServiceFault> e)
            {
                tracingService.Trace("Exception: {0}", e.ToString());
                tracingService.Trace("Timestamp: {0}", e.Detail.Timestamp);
                tracingService.Trace("Code: {0}", e.Detail.ErrorCode);
                tracingService.Trace("Message: {0}", e.Detail.Message);
                tracingService.Trace("Trace: {0}", e.Detail.TraceText);
                throw e;
            }