Help Adding item to existing Object List (ASP.NET MVC C#)

I'm try to make a little blog application in ASP.NET MVC3 with C#.

I have a BlogEntry Class and Comment Class.

public class BlogEntry    {

    public int Id { get; set; }

    public string Title { get; set; }

    public string Body { get; set; }

    public List<Comment> Comments { get; set; }

    public void开发者_开发百科 addComment(Comment comment)
    {
        Comments.Add(comment);
    }

}

I want to add a Comment to the existing Comment List for a particular Blog post. My Controller has the following code to add a Comment.

    [HttpPost]
    public ActionResult Comment(CommentViewModel commentViewModel)
    {
        if (ModelState.IsValid)
        {
            //Create New Comment
            Comment comment = new Comment();
            //Map New Comment to ViewModel
            comment.Title = commentViewModel.Title;
            comment.Message = commentViewModel.Message;
            comment.TimeStamp = DateTime.UtcNow;

            //Save newComment
            CommentDB.Comment.Add(comment);
            CommentDB.SaveChanges();

            //Get Entry by Id
            BlogEntry blogEntry = BlogDB.BlogEntry.Find(commentViewModel.BlogEntryId);
            // Add comment to Entry
            blogEntry.addComment(comment); // ERROR DISPLAYED HERE
            UpdateModel(blogEntry);
            BlogDB.SaveChanges();

            return RedirectToAction("Index");
        }
        else
        {
            return View(commentViewModel);
        }
    }

When I try to add a comment I get the following error: "Object reference not set to an instance of an object."

It seems like your Comments list isn't instantiated. Try something like this:

 public class BlogEntry
{
    public BlogEntry()
    {
        this.Comments = new List<Comment>();
    }

    public int Id { get; set; }

    public string Title { get; set; }

    public string Body { get; set; }

    public List<Comment> Comments { get; set; }

    public void addComment(Comment comment)
    {
        Comments.Add(comment);
    }

}