Error: Specified Cast is Not Valid

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VB

MS-Excel

When copying worksheets between workbooks in VB.net, I get this error:
Specified Cast is Not Valid

I inserted a message box MessageBox.Show(wshTest.Range("A8").Value.ToString) to verify that the worksheet being copied to the other workbook was valid. It was. I'm at my wits end.

wshTest.Copy(wbkDest.Sheets())

Private Sub btnGenerateReport_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGenerateReport.Click
        Dim dpl As New List(Of DatasetPair)
        'Try
        Me.Cursor = Cursors.WaitCursor
        Dim intSheetCount As Integer = 0
        For i As Integer = 0 To lvMatchedDvid.Items.Count - 1
            Dim dp As New DatasetPair
            dp.DataVersionIdOne = lvMatchedDvid.Items(i).Text.ToString
            dp.DataVersionIdTwo = lvMatchedDvid.Items(i).SubItems(1).Text.ToString
            intSheetCount += 1
            dp.SheetId = intSheetCount
            dpl.Add(dp)
        Next

        Dim wb As Excel.Workbook = Utilities.CreateWorkbook()
        Dim strFilePath As String = wb.Path.ToString & "\" & wb.Name.ToString

        For Each dp As DatasetPair In dpl
            Dim excelApp As New Excel.Application
            Dim ws As New Excel.Worksheet
            ws = GenerateWorksheet(dp)

            Dim strSourceFilePath As String
            strSourceFilePath = wb.Path.ToString & "\" & wb.Name.ToString
            Dim wbkDest As Excel.Workbook = excelApp.Workbooks.Open(strFilePath)
            Dim wshTest As Excel.Worksheet = DirectCast(ws, Excel.Worksheet)
            Dim wbkSource As Excel.Workbook = excelApp.Workbooks.Open(strSourceFilePath)
            MessageBox.Show(wshTest.Range("A8").Value.ToString)            
            wshTest.Copy(wbkDest.Sheets())            
        Next