Dynamics CRM 2011 Online - Invalid Argument

I have a custom workflow that seems to be working (it updates the lead) but receives an Invalid Argument error. After putting in various traces and getting no output from them I commented out all of the code that could be causing the problem. I still get the error message. The message text is...

Plugin Trace:

[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.RetrieveEntity]
[RetrieveEntity]

Error Message:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Expected non-empty Guid.Detail:
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147220989</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>Expected non-empty Guid.</Message>
  <Timestamp>2013-05-24T16:43:29.5240702Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147220970</ErrorCode>
    <ErrorDetails xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <Message>System.ArgumentException: Expected non-empty Guid.
Parameter name: id</Message>
    <Timestamp>2013-05-24T16:43:29.5240702Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText>[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.RetrieveEntity]
[RetrieveEntity]
</TraceText>
</OrganizationServiceFault>
   at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.Retrieve(String entityName, Guid id, ColumnSet columnSet, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType)
   at Microsoft.Crm.Extensibility.InprocessServiceProxy.RetrieveCore(String entityName, Guid id, ColumnSet columnSet)
   at Microsoft.Xrm.Sdk.Client.OrganizationServiceProxy.Retrieve(String entityName, Guid id, ColumnSet columnSet)
   at Microsoft.Crm.Workflow.Services.RetrieveActivityService.<>c__DisplayClass1.<RetrieveInternal>b__0(IOrganizationService sdkService)
   at Microsoft.Crm.Workflow.Services.ActivityServiceBase.ExecuteInTransactedContext(ActivityDelegate activityDelegate)
   at Microsoft.Crm.Workflow.Services.RetrieveActivityService.ExecuteInternal(ActivityContext executionContext, RetrieveEntity retrieveEntity)
   at Microsoft.Crm.Workflow.Services.RetrieveActivityService.Execute(ActivityContext executionContext, RetrieveEntity retrieveEntity)
   at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
   at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)

Here is the current code...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.Activities;
using System.Runtime.Serialization;

using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Messages;
using Microsoft.Xrm.Sdk.Workflow;
using Microsoft.Xrm.Sdk.Query;
using Microsoft.Crm.Sdk.Messages;

namespace CrmAssignTerritories
{
    public class AssignAccountTerritory : CodeActivity
    {
        [Input("string input")]
        public InArgument<string> ZipCode { get; set; }

        //[Output("EntityReference output only")]
        //[ReferenceTarget("systemuser")]
        //public OutArgument<EntityReference> Territory { get; set; }

        protected override void Execute(CodeActivityContext executionContext)
        {
            Boolean zipCodeFound = false;

            String searchZipCode;
            
            Guid territoryGuid = new Guid();
        
            IWorkflowContext workflowContext = executionContext.GetExtension<IWorkflowContext>();
            IOrganizationServiceFactory serviceFactory = executionContext.GetExtension<IOrganizationServiceFactory>();
            IOrganizationService service = serviceFactory.CreateOrganizationService(workflowContext.UserId);

            //Create the tracing service
            ITracingService tracingService = executionContext.GetExtension<ITracingService>();

            tracingService.Trace("Start Assign Account Territory");
/*
            if (workflowContext.PrimaryEntityName == "account" || workflowContext.PrimaryEntityName == "lead")
            {
                String zipCode = ZipCode.Get<string>(executionContext);

                searchZipCode = zipCode;
                int zipCodeLength = searchZipCode.Length;

                String entityName = workflowContext.PrimaryEntityName;
                Guid entityId = workflowContext.PrimaryEntityId;

                Entity updateEntity = new Entity(entityName);

                updateEntity.Id = entityId;
                
                do
                {
                    tracingService.Trace("Query zip code {0}", searchZipCode);

                    QueryByAttribute zipCodeQuery = new QueryByAttribute();

                    zipCodeQuery.ColumnSet = new ColumnSet("ree_territoryid");
                    zipCodeQuery.EntityName = "ree_territoryzipcode";
                    zipCodeQuery.AddAttributeValue("ree_zipcode", searchZipCode);

                    EntityCollection zipCodeCollection = service.RetrieveMultiple(zipCodeQuery);

                    foreach (var zipCodeEntity in zipCodeCollection.Entities)
                    {
                        tracingService.Trace("Zip code found");

                        territoryGuid = ((EntityReference)zipCodeEntity["ree_territoryid"]).Id;
                        zipCodeFound = true;
                    }

                    if (!zipCodeFound)
                    {
                        zipCodeLength--;
                        searchZipCode = searchZipCode.Substring(0, zipCodeLength);
                    }

                } while (zipCodeLength > 0 && !zipCodeFound);

                if (!zipCodeFound)
                {
                    tracingService.Trace("Zip code NOT found");
                    QueryByAttribute defaultTerritoryQuery = new QueryByAttribute();

                    defaultTerritoryQuery.ColumnSet = new ColumnSet("territoryid");
                    defaultTerritoryQuery.EntityName = "territory";
                    defaultTerritoryQuery.AddAttributeValue("name", "DEFAULT");

                    EntityCollection defaultTerritoryCollection = service.RetrieveMultiple(defaultTerritoryQuery);

                    foreach (var defaultTerritoryEntity in defaultTerritoryCollection.Entities)
                    {
                        tracingService.Trace("Default zip code found");

                        territoryGuid = defaultTerritoryEntity.Id;
                    }
                }

                updateEntity["gsi_territoryrsmid"] = new EntityReference("territory", territoryGuid);
                service.Update(updateEntity);
            } */
        }
    }
}

Gary