I have problem with inserting in data base

Cannot insert the value NULL into column 'Id', table 'HomeWorkSystem.dbo.CourseGroup'; column does not allow nulls. INSERT fails.
The statement has been terminated.

[Column(Storage="_Id", AutoSync=AutoSync.OnInsert, IsPrimaryKey=true, IsDbGenerated=true)]
public int Id
{
    get
    {
        return this._Id;
    }
    set
    {
        if ((this._Id != value))
        {
            this.OnIdChanging(value);
            this.SendPropertyChanging();
            this._Id = value;
            this.SendPropertyChanged("Id");
            this.OnIdChanged();
        }
    }
}