Observable Collection and INotifyPropertyChange + NullReferenceException problem

Question

hi, What's wrong with this code? An exception of type 'System.NullReferenceException' on "eventHandler(this, new PropertyChangedEventArgs(propertyname));" line.


File City.cs

namespace Banks.Classes
{
    public class City : INotifyPropertyChanged
    {
        // name city
        private string _name = string.Empty;

        public string Name
        {
            get
            {
                return this._name;
            }
            set
            {
                if (this._name != value)
                {
                    this._name = value;
                    OnPropertyChanged("Name");
                }
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyname)
        {
            var eventHandler = this.PropertyChanged;
            if (eventHandler == null)
            {
                eventHandler(this, new PropertyChangedEventArgs(propertyname));
            }
        }
    }
}



File MainPage.xaml.cs

namespace Banks
{

    public sealed partial class MainPage : Page
    {

        public ObservableCollection<City> cities { get; set; }

        public MainPage()
        {
            this.InitializeComponent();

            this._loadCities();
        }

        private void _loadCities()
        {
            City city = new City();
            city.Name = "new york";
            cities.Add(city);
        }

    }

}