¿Por qué no puedo pasar la lista <Customer> como parámetro a un método que acepta la lista <object>?

Question

El siguiente código me da este error:

Cannot convert from 'System.Collections.Generic.List' to 'System.Collections.Generic.List'.

¿Cómo puedo indicar al compilador que el Cliente de hecho hereda de objeto? ¿O simplemente no hereda con objetos de colección genéricos (enviar un List<string> obtiene el mismo error).

using System.Collections.Generic; 
using System.Windows; 
using System.Windows.Documents; 

namespace TestControl3423 
{ 
    public partial class Window2 : Window 
    { 
     public Window2() 
     { 
      InitializeComponent(); 

      List<Customer> customers = Customer.GetCustomers(); 
      FillSmartGrid(customers); 

      //List<CorporateCustomer> corporateCustomers = CorporateCustomer.GetCorporateCustomers(); 
      //FillSmartGrid(corporateCustomers); 
     } 


     public void FillSmartGrid(List<object> items) 
     { 
      //do reflection on items and display dynamically 
     } 
    } 

    public class Customer 
    { 
     public string FirstName { get; set; } 
     public string LastName { get; set; } 
     public string Street { get; set; } 
     public string Location { get; set; } 
     public string ZipCode { get; set; } 

     public static List<Customer> GetCustomers() 
     { 
      List<Customer> customers = new List<Customer>(); 
      customers.Add(new Customer { FirstName = "Jim", LastName = "Jones", ZipCode = "23434" }); 
      customers.Add(new Customer { FirstName = "Joe", LastName = "Adams", ZipCode = "12312" }); 
      customers.Add(new Customer { FirstName = "Jake", LastName = "Johnson", ZipCode = "23111" }); 
      customers.Add(new Customer { FirstName = "Angie", LastName = "Reckar", ZipCode = "54343" }); 
      customers.Add(new Customer { FirstName = "Jean", LastName = "Anderson", ZipCode = "16623" }); 
      return customers; 
     } 
    } 
}