Object reference not set error when setting specialPermissions in createtask activity for sequential workflow

Hi,

I am trying to use the SpecialPermissions in CreateTask Activity. I am trying to set the custom role. The custom role exists in the site.

I have tried the below code:

private void createTaskforApprover_MethodInvoking(object sender, EventArgs e)
 {
 this.ApproverTaskId = Guid.NewGuid();
 CreateTask taskForApprover = sender as CreateTask;
using (SPSite oSite = new SPSite(workflowProperties.SiteId))
            {
                using (SPWeb oWeb = oSite.OpenWeb(workflowProperties.WebId))
                {
                    oWeb.AllowUnsafeUpdates = true;
                    SPRoleDefinition oSPRoleDefinition = oWeb.RoleDefinitions["customRoleName"];
                    taskForApprover.SpecialPermissions.Add("domain\userloginname, oSPRoleDefinition);
                    oWeb.AllowUnsafeUpdates = false;
                }
            }
}

There is value in the oSPRoleDefinition. I am getting the object reference  error in the SpecialPermissions.Add.

How to fix this? How to add the custom role in special permissions?

Thanks