Compare two excel files using ClosedXML

[luckydead]

I have issue with the code. I receive error message: System.IndexOutOfRangeException: 'Cannot find column 21.'
and it stops here
dt.Rows(dt.Rows.Count - 1)(i) = cell.Value.ToString()

The column exists in file, but it gives error.
Using ClosedXML 0.96.0 version for .net framework 4.5

Private Function ReadExcelToDataTable(ByVal filePath As String) As DataTable
        Dim dt As DataTable = New DataTable()
        Using workBook As XLWorkbook = New XLWorkbook(filePath)
            Dim workSheet As IXLWorksheet = workBook.Worksheet(1)
            Dim firstRow As Boolean = True
            For Each row As IXLRow In workSheet.Rows()
                If firstRow Then
                    For Each cell As IXLCell In row.Cells()
                        dt.Columns.Add(cell.Value.ToString())
                    Next
                    firstRow = False
                Else
                    dt.Rows.Add()
                    Dim i As Integer = 0
                    For Each cell As IXLCell In row.Cells()
                        dt.Rows(dt.Rows.Count - 1)(i) = cell.Value.ToString()
                        i += 1
                    Next
                End If
            Next
        End Using

        Return dt
    End Function

Dim file1 As String = System.AppDomain.CurrentDomain.BaseDirectory & "\Production modules.xlsx"
        Dim file2 As String = System.AppDomain.CurrentDomain.BaseDirectory & "\ConnectorListMST.xlsx"
        Using dt1 As DataTable = ReadExcelToDataTable(file1)
            Using dt2 As DataTable = ReadExcelToDataTable(file2)

                Dim partNos = (From data In dt2.AsEnumerable()
                               Select data("Part No")).Distinct().ToArray()
                Dim dt As DataTable = New DataTable()
                dt.Columns.Add("Partnumber")
                dt.Columns.Add("Status")

                For Each dr As DataRow In dt1.Rows
                    If dr("Note").ToString().ToLower().Contains("connector") Then
                        If Not String.IsNullOrEmpty(dr("Yazaki name").ToString()) Then
                            Dim isExist = (From data In dt.AsEnumerable()
                                           Where data("Partnumber").Equals(dr("Yazaki name"))
                                           Select data("Partnumber")).ToArray()
                            If isExist.Length = 0 Then
                                If Array.IndexOf(partNos, dr("Yazaki name").ToString()) < 0 Then
                                    dt.Rows.Add(dr("Yazaki name").ToString(), "missing number %s")
                                Else
                                    dt.Rows.Add(dr("Yazaki name").ToString(), "Exists")
                                End If
                            End If
                        End If
                    End If
                Next

                DataGridView3.DataSource = dt
            End Using
        End Using