Create Employee and set Role using C#

I'm new to NetSuite development and I'm trying to use SuiteTalk to create a new employee record and set the employee's role.

The snippet of C# code that I'm using is:

   var employee = new Employee
   {
      firstName = "First",
      lastName = "Last",
      email = "...",
      giveAccess = true,
      giveAccessSpecified = true,
      password = "***",
      password2 = "***"
   };

   var rolesList = new EmployeeRolesList
   {
      roles = new EmployeeRoles[]
      {
         new EmployeeRoles{ selectedRole = new RecordRef { internalId = "18", type = RecordType.salesRole, typeSpecified = true }},
      }
   };
   employee.rolesList = rolesList;

   var response = session.Add(employee);

The status field in the response has a code of  612 and messsage of "Invalid selectedrole reference key 18".

Can someone point me towards the correct way to assign the roles to the new employee record?