Passing email entity to custom workflow activity

Hi, I am looking to pass the email entity reference as the input parameter for the custom workflow activity, but I got the below error, please advise.

[Input("Email")]
        [ReferenceTarget("activityid")]
        [RequiredArgument]
        public InArgument<EntityReference> Email { get; set; }
        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("start getting the email entity");
            Entity EmailEntity = service.Retrieve("email", Email.Get<EntityReference>(executionContext).Id, new ColumnSet(true));
            tracingService.Trace("got email entity");
            tracingService.Trace("Subject",EmailEntity.GetAttributeValue<string>("subject"));
            tracingService.Trace("Description",EmailEntity.GetAttributeValue<string>("description"));
        }