Object reference not set to an instance of an object.

I keep getting an error when I try to invoke a class in my VB 2005 project.  What follows is my code:
Public Class OpticalDensity
    Public Key As Double
    Public Value As Double
    Public Log10 As Double
    Public Ln As Double
    Public Log10LnProd As Double
    Public Result As Double

    Public Sub New(ByVal dblKey As Double, ByVal dblValue As Double, Optional ByVal dblLnSub As Double = 0)
        Key = dblKey
        Value = dblValue
        Log10 = Math.Log(Key) / Math.Log(10.0#)
        Ln = Math.Log(dblLnSub / (1 - dblLnSub)) / Math.Log(2.718281828)
        Log10LnProd = Log10 * Ln
    End Sub
End Class
---------------------------------------------------------------------------------------------
Public Class OpticalDensities
    Private mCol As Collection
    Protected Friend ZeroOD As Double
...
---------------------------------------------------------------------------------------------
    Public Sub Add(ByVal dblKey As Double, ByVal dblValue As Double)
        Dim od As New OpticalDensity(dblKey, dblValue, dblValue / ZeroOD)
        mCol.Add(od)  'HERE IS WHERE I GET THE ERROR
...
---------------------------------------------------------------------------------------------

Thanks for your help.

brightak