Plugin: Input string not in the correct format

Hi, 

The following plugin, firing on create of the invoice line entity, fails with the error message: <Message>An unexpected error occurred from ISV code. (ErrorType = ClientError) Unexpected exception from plug-in (Execute): InvoiceLineCalculation.InvoiceLineCalculation: System.Exception: Input string was not in a correct format.</Message>. Do you know what might cause this issue?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.ServiceModel;
using Microsoft.Xrm.Sdk;

namespace InvoiceLineCalculation
{
   public class InvoiceLineCalculation : IPlugin
    {
        public void Execute(IServiceProvider serviceProvider)
        {
            // Obtain the tracing service
            ITracingService tracingService =
            (ITracingService)serviceProvider.GetService(typeof(ITracingService));

            // Obtain the execution context from the service provider.  
            IPluginExecutionContext context = (IPluginExecutionContext)
                serviceProvider.GetService(typeof(IPluginExecutionContext));

            // The InputParameters collection contains all the data passed in the message request.  
            if (context.InputParameters.Contains("Target") &&
                context.InputParameters["Target"] is Entity)
            {
                // Obtain the target entity from the input parameters.  
                Entity entity = (Entity)context.InputParameters["Target"];

                // Obtain the organization service reference which you will need for  
                // web service calls.  
                IOrganizationServiceFactory serviceFactory =
                    (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
                IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

                try
                {
                    if (entity.Attributes.Contains("priceperunit")&& entity.Attributes.Contains("volumediscountamount")&& entity.Attributes.Contains("quantity")&& entity.Attributes.Contains("manualdiscountamount")&& entity.Attributes.Contains("tax"))
                    {
                        decimal pricePerUnit = Convert.ToDecimal(entity["priceperunit"].ToString());
                        decimal volumeDiscount = Convert.ToDecimal(entity["volumediscountamount"].ToString());
                        decimal quantity = Convert.ToDecimal(entity["quantity"].ToString());
                        decimal discountAmount = Convert.ToDecimal(entity["manualdiscountamount"].ToString());
                        decimal tax = Convert.ToDecimal(entity["tax"].ToString());

                        decimal calculation = (((pricePerUnit - volumeDiscount) * quantity) - discountAmount + tax);
                        entity["extendedamount"] = calculation;
                    }
                }
                catch (FaultException<OrganizationServiceFault> ex)
                {
                    throw new InvalidPluginExecutionException("An error occurred in InvoiceLineCalculation Plugin.", ex);
                }

                catch (Exception ex)
                {
                    tracingService.Trace("InvoiceLineCalculation: {0}", ex.ToString());
                    throw;
                }
            }
        }
    }
}