Modifying a Plugin to Sum Values of a Record and populate the total

Hello, 

I have an entity called new_accrued_billing that I've setup as a sub grid in the project module so you can click in the grid from within a project and create a record. My next requirement involves summing the values of 1 field in the Subgrid Records and summing them into a field on the project module. I've modified an existing plugin (that does the same thing for Time Entries on a project). 

 decimal Billed = 0;
               // decimal NonBillable_Total = 0;
                lastAction += "\nRun Totals For loop";
                if (Project_Billing != null)
                {
                    lastAction += "\nTime Entry is not null " + Project_Billing.Count.ToString();
                    if (Project_Billing.Count > 0)
                    {
                        lastAction += "\nCount is not null eric " + Project_Billing.Count.ToString();
                        foreach (Entity Project_Billings in Project_Billing)
                        {
                          Billed += (Convert.ToDecimal(Project_Billings["new_billed"].ToString()));
                        }
                     }
                 }

                Entity updateProject = new Entity();
                updateProject.LogicalName = "new_project";
                updateProject.Id = parentEvent.Id;
                updateProject["new_billing_summary"] = Convert.ToDecimal(Billed.ToString());
               // updateProject["new_nonbillabletime"] = Convert.ToDecimal(NonBillable_Total.ToString());
                lastAction += "\nAttempting Update " + updateProject.Id.ToString();

                try 
                {
                    crmService.Update(updateProject);
                }
                catch (FaultException e)
                {
                    lastAction += "\nError: " + e.Message;
                    throw new InvalidPluginExecutionException("Fault Exception: " + lastAction);
                }

	}//try
                try 
                {
                    crmService.Update(updateProject);
                }
                catch (FaultException e)
                {
                    lastAction += "\nError: " + e.Message;
                    throw new InvalidPluginExecutionException("Fault Exception: " + lastAction);
                }

	}//try

It compiles successfully and registers however I receive the following error:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Catch All Exception: 
tracingService setup for Project_Billing_Rollup
serviceFactory set
service set
MessageName = Create
Create Message
Comes to Billing Entry
Parent Obtained 
Run Totals For loop
Time Entry is not null 3
Count is not null eric  3Detail: 
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147220891</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic">
    <KeyValuePairOfstringanyType>
      <d2p1:key>OperationStatus</d2p1:key>
      <d2p1:value xmlns:d4p1="http://www.w3.org/2001/XMLSchema" i:type="d4p1:string">0</d2p1:value>
    </KeyValuePairOfstringanyType>
  </ErrorDetails>
  <Message>Catch All Exception: 
tracingService setup for Project_Billing_Rollup
serviceFactory set
service set
MessageName = Create
Create Message
Comes to Billing Entry
Parent Obtained 
Run Totals For loop
Time Entry is not null 3
Count is not null eric  3</Message>
  <Timestamp>2013-05-02T20:29:43.7746617Z</Timestamp>
  <InnerFault i:nil="true" />
  <TraceText>

[Billing_Summary_Rollup: Project_Billing_Rollup.Project_Billing_Rollup]
[ Project_Billing_Rollup.Project_Billing_Rollup: Create of new_accrued_billing]
</TraceText>
</OrganizationServiceFault>

Any suggestions would be great! 

Thanks!

Eric