Unable to register custom workflow using plugin registration tool,The type Int32 of the property is not supported in CRM 4.0.

I have created a model property in my custom workflow code, generated assembly of the code. I have then tired registering of the assembly file usingplug-in registration tool but at the end of completion getting an error message like below- The type Int32 of the property model is not supported. (Is plug-in registration tool supports 64-bit int type, if then how to change the int32 to int64 from the code I have pasted in bottom)

Unhandled Exception: System.Web.Services.Protocols.SoapException: Server was unable to process request.
Detail: <detail><error>
  <code>0x8004501d</code>
  <description>The type Int32 of the property model is not supported.</description>
  <type>Platform</type>
</error></detail>
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at PluginRegistrationTool.CrmSdk.CrmService.Create(BusinessEntity entity)
   at PluginRegistrationTool.RegistrationHelper.RegisterPluginType(CrmOrganization org, CrmPlugin plugin)
   at PluginRegistrationTool.PluginRegistrationForm.btnRegister_Click(Object sender, EventArgs e)

Please refer my code below and correct me if I did something wrong.

[CrmWorkflowActivity("Create Cardetails Record", "Utilities")]
    public partial class CreateCardetails : SequenceActivity
    {
        public static DependencyProperty modelProperty = DependencyProperty.Register("model", typeof(int), typeof(CreateCardetails));

        [CrmInput("Model")]
        public int model
        {

            get
            {
                return (int)base.GetValue(modelProperty);
            }

            set
            {
                base.SetValue(modelProperty, value);
            }

        }

        public CreateCardetails()
        {

            InitializeComponent();

        }

        protected override ActivityExecutionStatus Execute(ActivityExecutionContext executionContext)
        {

            // Get the context service.
            IContextService contextService = (IContextService)executionContext.GetService(typeof(IContextService));
            IWorkflowContext context = contextService.Context;

            // Use the context service to create an instance of CrmService.
            ICrmService crmService = context.CreateCrmService(true);

            DynamicEntity entity = null;
            Guid contactId;

            if (context.InputParameters.Properties.Contains("Target") && context.InputParameters.Properties["Target"] is DynamicEntity)
            {
                // Obtain the target business entity from the input parmameters.
                entity = (DynamicEntity)context.InputParameters.Properties["Target"];
                contactId = ((Key)entity.Properties["contactid"]).Value;

                Lookup lookup = new Lookup();
                lookup.Value = contactId;
                lookup.type = "contact";

                //Create an account which is linked to the contact record
                DynamicEntity cardetails = new DynamicEntity("cir_cardetails");

                cardetails["cir_carsdetailsid"] = lookup;

                //Setting the picklist value of Model
                Picklist modelPickList = new Picklist();
                modelPickList.Value = (int)model;
                cardetails.Properties.Add(new PicklistProperty("cir_model", modelPickList));
                Guid carkey = crmService.Create(cardetails);
                 
            }
            return ActivityExecutionStatus.Closed;
        }
    }

Kindly help, my all lucks are now gone. I have no idea how to fix it and even not sure about the issue.