Object reference not set to an instance of an object.

Public Class Form1
    Dim WithEvents Monitor As Timer
    Dim Drives As Dictionary(Of Integer,  System.IO.DriveInfo)

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        For Letters As Int16 = 65 To 90
            Dim mDrives As System.IO.DriveInfo = New System.IO.DriveInfo(Chr(Letters))
            Drives.Add(Letters, mDrives) '--> Getting the error here.
        Next
        Monitor.Start()
    End Sub
end class