VB.NET2005: NeroISOTrack in an array


I am developing an application which will scan a directory and add files as needed and then burning them to a CD.

Adding of teh files is fine but I seem to run accross a problem in determining how many cd is needed. What I basically doing is scan the directories and add the files to a datatable with its path and size. I then add one file at a time from the data table to neroisotrack until the total size of files reaches the maximum disc size. I then create a new neroisotrack in the array.

Can anyone tell me if this work or is there a better way of doing things?
Here is the basic flow.

    Dim maxDiscLength As Long = 650000000
    Dim neroTrack() As NeroISOTrack

'Add Files to ISOTrack til disc reaches maxDiscLength

Assign ISO Track 
neroTrack(0) = isotrack

'Add More Files to ISOTrack til disc reaches maxDiscLength
neroTrack(1) = isotrack

My problem is if i do MessageBox.Show(neroTrack.Length) to determine how many track was created. I get error object refrence not set to an instance of an object.