Nullable DateTime conversion 可空 DateTime 转换

我试图将我的reader[3]对象从datetime转换为null，如果一个论坛没有lastPostDate，但是它说我缺少转换。错误：条件表达式的类型无法确定，因为和'System.DateTime'之间没有隐式转换。

public class Forums
{
    public List GetForums()
    {
        using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["CMS"].ConnectionString))
        {
            conn.Open();
            SqlCommand cmd = new SqlCommand("sproc_Forums_GetForums", conn);
            cmd.CommandType = CommandType.StoredProcedure;
            SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.Default);
            List forums = new List();
            while (reader.Read())
            {
                var title = reader[6].ToString();
                var threadCount = (int)reader[5];
                var lastPostTitle = reader[4].ToString();
                // 下面是我遇到问题的地方
                Nullable lastPostDate = (reader[3] == DBNull.Value ? null : Convert.ToDateTime(reader[3])); 
                var lastPostBy = reader[2].ToString();
                var forumGroup = reader[1].ToString();
                var description = reader[0].ToString();
                Forum forum = new Forum(0, "",DateTime.Now,
                    reader["Title"].ToString(),description,
                    0,false,"","",DateTime.Now,true,
                    forumGroup, (int)threadCount, lastPostBy,
                    lastPostDate, lastPostTitle);
                forums.Add(forum);/**/
            }
            return forums;
        }
    }            
}
public class Forum
{
    public Forum(int forumID, string addedBy, DateTime addedDate, string title, string description, int parentID, bool moderated,
        string imageUrl, string updatedBy, DateTime? updatedDate, bool active, string forumGroup, int threadCount, string lastPostBy,
        Nullable lastPostDate, string lastPostTitle)
    {
        this.ForumID = forumID;
        this.AddedBy = addedBy;
        this.AddedDate = addedDate;
        this.Title = title;
        this.Description = description;
        this.ParentID = parentID;
        this.Moderated = moderated;
        this.ImageUrl = imageUrl;
        this.UpdatedBy = updatedBy;
        this.UpdatedDate = updatedDate;
        this.Active = active;
        this.ForumGroup = forumGroup;
        this.ThreadCount = threadCount;
        this.LastPostBy = lastPostBy;
        this.LastPostDate = lastPostDate;
        this.LastPostTitle = lastPostTitle;
    }
    private int _forumID;
    public int ForumID
    {
        get { return _forumID; }
        set { _forumID = value; }
    }
    private string _addedBy;
    public string AddedBy
    {
        get { return _addedBy; }
        set { _addedBy = value; }
    }
    private DateTime _addedDate = DateTime.Now;
    public DateTime AddedDate
    {
        get { return _addedDate; }
        set { _addedDate = value; }
    }
    private string _title = "";
    public string Title
    {
        get { return _title; }
        set { _title = value; }
    }
    private string _description = "";
    public string Description
    {
        get { return _description; }
        set { _description = value; }
    }
    private int _parentID = 0;
    public int ParentID
    {
        get { return _parentID; }
        set { _parentID = value; }
    }
    private bool _moderated = false;
    public bool Moderated
    {
        get { return _moderated; }
        set { _moderated = value; }
    }
    private string _imageUrl = "";
    public string ImageUrl
    {
        get { return _imageUrl; }
        set { _imageUrl = value; }
    }
    private string _updatedBy = "";
    public string UpdatedBy
    {
        get { return _updatedBy; }
        set { _updatedBy = value; }
    }
    private DateTime? _updatedDate = null;
    public DateTime? UpdatedDate
    {
        get { return _updatedDate; }
        set { _updatedDate = value; }
    }
    private bool _active = false;
    public bool Active
    {
        get { return _active; }
        set { _active = value; }
    }
    private string _forumGroup = "";
    public string ForumGroup
    {
        get { return _forumGroup; }
        set { _forumGroup = value; }
    }
    private int _threadCount = 0;
    public int ThreadCount
    {
        get { return _threadCount; }
        set { _threadCount = value; }
    }
    private string _lastPostBy = "";
    public string LastPostBy
    {
        get { return _lastPostBy; }
        set { _lastPostBy = value; }
    }
    private Nullable _lastPosteDate = null;
    public Nullable LastPostDate
    {
        get { return _lastPosteDate; }
        set { _lastPosteDate = value; }
    }
    private string _lastPostTitle = "";
    public string LastPostTitle
    {
        get { return _lastPostTitle; }
        set { _lastPostTitle = value; }
    }
}