Appointment Entity inheritance on client

[NickName]

I have implemented my class Meeting which is partial class and inherits from
Entity class named Appointment:

public abstract partial class Appointment : Entity{....}

Partial Meeting class is also entity:

public sealed partial class Meeting : Appointment { ... }

Second Meeting class implementation I made similar to implementation of Telerik Appointment class which inherits from AppointmentBase.

The issue raise when I try to create my Meeting class instance:

private void btnCreateMeeting_Click(object sender, System.Windows.RoutedEventArgs e)
{
      var meeting = new Meeting
      {
         Start = DateTime.Now,
         End = DateTime.Now.AddHours(2)
      };
      RadScheduleViewCommands.CreateAppointment.Execute(meeting, scheduleView);
}

Appointment dialog box appears. I enter values and click OK and get error:

Message: System.ArgumentOutOfRangeException: Index was out of range.
Must be non-negative and less than size of the collection
Parameter name: index

I don`t even know what the issue is? In what property? Can you help me or direct me in some right way.
Thank you!

[NickName]

Can anybody help me? Thanks...