Problem using List Except

[thomus07]

 interface IValidationData : IEquatable<IValidationData>
        {
       
        }       

        class Customer : IValidationData
        {
            public string Name { get; set; }
            public int Age { get; set; }            

            public bool Equals(IValidationData otherValue)
            {
                Customer other = otherValue as Customer;

                if (!Name.Equals(other.Name))
                    return false;
                if (!Age.Equals(other.Age))
                    return false;

                return true;
            }

            public override int GetHashCode()
            {
                return Name.GetHashCode() ^ Age.GetHashCode();
            }
        }

    List<Customer> list1 = new List<Customer>();
    List<Customer> list2 = new List<Customer>(); 
    List<Customer> list3 = list2.Except(list1).ToList(); 
// This line is not working why?

I can't use Except method of List, why?