Creating a temporary datatable and adding columns

Experts,

For several years I have been using the following code without issue:

        Dim dtDataTable As DataTable = New DataTable
        Dim iRow As Integer = 0
        Dim iCol As Integer = 0
        Dim dtCol() As DataColumn = Nothing
        Dim oRow As DataRow = Nothing
        For iLoop As Integer = 1 To 3
            dtCol(iLoop - 1) = New DataColumn
            dtDataTable.Columns.Add(dtCol(iLoop - 1))
        Next iLoop

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For some reason, however, today I encountered a Null Reference exception on the line:
"dtCol(iLoop - 1) = New DataColumn"

Earlier in the code I have identified dtCol() as a DataColumn and assigned Nothing to it until I needed to.  This is really weird.

I am now using VisualStudio Community 2015 with .Net 4.6.  Am I mussing something.  I bet ots going to be really easy, but I can't see it for some reason.  Your help would be appreciated.  Thank you