Error: CallExternalMethodActivity parameter specialPermissions not serializable in workflow

HI,

I have a custom sequential workflow created in VS 2010. I am trying to set custom role on create task special permissions.

I have used the below code:

public HybridDictionary createTask_SpecialPermissions;
  private void createTaskforApprover_MethodInvoking(object sender, EventArgs e)
        {
            this.ApproverTaskId = Guid.NewGuid();
            CreateTask taskForApprover = sender as CreateTask;
            this.TaskPropertiesForApprover.AssignedTo = "domain\approveruserid";
            this.TaskPropertiesForApprover.TaskType = 1;
   	using (SPSite oSite = new SPSite(workflowProperties.SiteId))
            {
                using (SPWeb oWeb = oSite.OpenWeb(workflowProperties.WebId))
                {
                    oWeb.AllowUnsafeUpdates = true;
                    SPRoleDefinition oSPRoleDefinition = oWeb.RoleDefinitions["custom role"];
                    SPRoleAssignment oSPRoleAssignmentManager = new SPRoleAssignment("domain\approveruserid", "", "", "");
                    oSPRoleAssignmentManager.RoleDefinitionBindings.Add(oSPRoleDefinition);
                    createTask_SpecialPermissions = new HybridDictionary();
                    createTask_SpecialPermissions.Add("domain\approveruserid", oSPRoleAssignmentManager);
                    taskForApprover.SpecialPermissions = createTask_SpecialPermissions;
                }
            }
            taskForApprover.TaskProperties = TaskPropertiesForApprover;
        }  

I am getting the error: CallExternalMethodActivity parameter specialPermissions not serializable

Stack Trace: at System.Workflow.Activities.InvokeHelper.CloneOutboundValue(Object source, BinaryFormatter formatter, String name)
   at System.Workflow.Activities.InvokeHelper.GetParameters(MethodBase methodBase, WorkflowParameterBindingCollection parameterBindings, ParameterModifier[]& parameterModifiers)
   at System.Workflow.Activities.CallExternalMethodActivity.Execute(ActivityExecutionContext executionContext)
   at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(T activity, ActivityExecutionContext executionContext)
   at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(Activity activity, ActivityExecutionContext executionContext)
   at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)
   at System.Workflow.Runtime.Scheduler.Run()

How to fix this?

Thanks