I´m Trying to Write a String in a Field C#

using System;
using System.Activities;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Workflow;
using Microsoft.Xrm.Sdk.Query;
using System.Text.RegularExpressions;

namespace ActivityLibrary1
{
    public sealed class KleinerTest : CodeActivity
    {
        protected override void Execute(CodeActivityContext executionContext)
        {
            ITracingService tracingService = executionContext.GetExtension<ITracingService>();
            IWorkflowContext context = executionContext.GetExtension<IWorkflowContext>();
            IOrganizationServiceFactory serviceFactory = executionContext.GetExtension<IOrganizationServiceFactory>();
            IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);
            tracingService.Trace("test");
            Guid ID = context.PrimaryEntityId;
            Entity Entity = service.Retrieve("account", ID, new ColumnSet(true));
            string input = "Testobjekt";
            Entity["new_historie"] = input;
            service.Update(Entity);
        }
    }
}

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Error:

Unexpected exception from plug-in (Execute): ActivityLibrary1.KleinerTest: System.AggregateException: One or more errors occurred.

Unhandled Exception: Microsoft.Crm.CrmException: Unexpected exception from plug-in (Execute): ActivityLibrary1.KleinerTest: System.AggregateException: One or more errors occurred.
   at Microsoft.Crm.Sandbox.SandboxCodeUnit.Execute(IExecutionContext context)
   at Microsoft.Crm.Workflow.Services.ProxyCustomActivity.Execute(CodeActivityContext executionContext)