Odd runtime error with custom workflow activity

namespace WorkflowFormatDateOnlyFromDateTime
{
    public class WorkflowFormatDateOnlyFromDateTime : CodeActivity
    {
        //Set input parameter
        [RequiredArgument]
        [Input("Date Time value")]
        //[Default(null)]
        public InArgument<DateTime> dateTimeValue { get; }
        [Output("Date Only Equivalent")]
        [Default("")]
        public OutArgument<string> dateOnlyValue { get; set; }

        //Main procedure
        protected override void Execute(CodeActivityContext context)
        {
            //Create a tracing service for debugging purposes
            ITracingService tracingService = context.GetExtension<ITracingService>();
            tracingService.Trace("Starting workflow activity to get Date Only from DateTime field.");

            //Get the workflow context from the CodeActivity context
            IWorkflowContext workflowContext = context.GetExtension<IWorkflowContext>();

            //Create an organization service
            tracingService.Trace("Getting Organization Service Factory object.");
            IOrganizationServiceFactory serviceFactory = context.GetExtension<IOrganizationServiceFactory>();
            tracingService.Trace("Creating Organization Service.");
            IOrganizationService service = serviceFactory.CreateOrganizationService(workflowContext.UserId);

            //Get the date only from a DateTime field
            *****DateTime TempDateTime = dateTimeValue.Get(context);*****
            tracingService.Trace("Date Time is " + TempDateTime.ToString());
            string TempDateOnly = TempDateTime.ToString("mm/dd/yyyy");
            tracingService.Trace("Temp date only is " + TempDateOnly);
            dateOnlyValue.Set(context, TempDateOnly);
        }
    }
}