Null Exception while loading my file to listbox. - vb.net

Question

I have a problem. I'm trying to load data to listbox , but when I click start I get an unhandled exception that states values cannot be null. I've bolded where the issue is.
I'm trying to create a library database, and would really appreciate if someone could help me fix this issue. Thanks!
Public Class frmLibrary
Structure BookData
Dim Title As String
Dim Author As String
Dim ISBN As Integer
Dim YearPublished As Integer
End Structure
Dim bookinfo() As BookData 'Array
Private Sub frmLibrary_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim bookfiles() As String = IO.File.ReadAllLines("LibraryDatabase.txt")
Dim n As Integer = bookfiles.Count - 1
ReDim bookinfo(n)
Dim line As String
Dim books() As String 'books are the parts
'use the split methods to assign values to the members of the structure variable.
For i As Integer = 0 To n ---**>Why is this value null???????**
line = bookfiles(i)
books = line.Split(","c) '
bookinfo(i).Title = books(0)
bookinfo(i).Author = books(1)
bookinfo(i).ISBN = books(2)
bookinfo(i).YearPublished = books(3)
Next
Dim query = From book In bookinfo
Select book.Title, book.Author, book.ISBN, book.YearPublished
DGVBookInfo.DataSource = query.ToList
DGVBookInfo.Columns("Title").HeaderText = "Title"
DGVBookInfo.Columns("Author").HeaderText = "Author"
DGVBookInfo.Columns("ISBN").HeaderText = "ISBN"
DGVBookInfo.Columns("Year Published").HeaderText = "Year Published"
DGVBookInfo.AutoResizeColumns()
DGVBookInfo.RowHeadersVisible = False
End Sub