Passing parameter to the SmartObject

Greetings,

The below mentioned code is throwing an exception."SmartObject property Role_Name is a required property for selected method Get_Role_Users. Value must be set"

Is this the right way to pass the parameter value to the SmartObject method ?

            SmartObject employee = serverName.GetSmartObject("UMUser");
            employee.MethodToExecute = "Get_Role_Users";
            employee.Properties[0].Value = "Preparer";
            SmartObjectList employees = serverName.ExecuteList(employee);

            foreach (SmartObject so in employees.SmartObjectsList)
            {
                string bar = so.Properties["Name"].Value.ToString().Trim();
                Console.WriteLine(bar);
            }

If I mention the property name as; exception is thrown as"Key could not be found in the collection ! "

            employee.Properties["Role_Name"].Value = "Preparer";

Can anyone let me know how to pass parameter to the smartObject? When I execute the same method through "SmartObject Service Tester.exe", it works fine.

Thanks,

Aman