error: "Type Excel.worksheet is not defined"

Private Sub excelsaveBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles excelsaveBtn.Click
       'Export the listview to an Excel spreadsheet
       SaveFileDialog1.Title = "Save Excel File"
       SaveFileDialog1.Filter = "Excel files (*.xls)|*.xls|Excel Files (*.xlsx)|*.xslx"
       SaveFileDialog1.ShowDialog()
       'exit if no file selected
       If SaveFileDialog1.FileName = "" Then
           Exit Sub
       End If
       'create objects to interface to Excel
       Dim xls As New Microsoft.Office.Interop.Excel.Application
       Dim book As Microsoft.Office.Interop.Excel.Workbook
       Dim sheet As Microsoft.Office.Interop.Excel.Worksheet
       'create a workbook and get reference to first worksheet
       xls.Workbooks.Add()
       book = xls.ActiveWorkbook
       sheet = CType(book.Sheets(1), Excel.Worksheet)

       'step through rows and columns and copy data to worksheet
       Dim row As Integer = 1
       Dim col As Integer = 1
       For Each item As ListViewItem In lvw.Items
           For i As Integer = 0 To item.SubItems.Count - 1
               sheet.Cells(row, col) = item.SubItems(i).Text
               col = col + 1
           Next
           row += 1
           col = 1
       Next
       'save the workbook and clean up
       book.SaveAs(SaveFileDialog1.FileName)
       xls.Workbooks.Close()
       xls.Quit()

   End Sub