PageRequestManagerParserErrorException: ... when trying to create an Excel Spread Sheet from ASP.Net

The message received from the server could not be parsed.  Common causes for this error when the response is modifiedy by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.

Details: Error parsing near 'Sample ID        First Crop'.

What I am trying to do from an ASP.Net (3.5) with AJAX update panel is when the user selects 'Export to Excel and presses the button.  Here is the code.  If there is another/better way to do this please advise.  This code I inherited.

 Response.Clear()
                Response.AddHeader("content-disposition", "attachment;filename=mySubmission.xls")
                Response.Charset = "UTF-8"
                Response.ContentType = "application/vnd.xls"

                ' getting heading
                Dim strValues As String = ""
                For ilcv As Integer = 1 To gvSamples.HeaderRow.Cells.Count - 2
                    strValues = strValues & gvSamples.HeaderRow.Cells(ilcv).Text & Chr(9)
                Next

                strValues = strValues & Chr(13)

                'read data from gridView
                For iLcv As Integer = 0 To gvSamples.Rows.Count - 1
                    strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(1).FindControl("txtSampleID"), TextBox).Text & Chr(9)
                    strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(3).FindControl("ddlFirstCode"), DropDownList).Text & Chr(9)
                    strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(5).FindControl("ddlSecondCode"), DropDownList).Text & Chr(9)

                    strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(6).FindControl("txtTonsAcre"), TextBox).Text & Chr(9)
                    
                    If CType(gvSamples.Rows(iLcv).Cells(7).FindControl("ddlLimeMonth"), DropDownList).Text <> "Please select:" Then
                        strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(7).FindControl("ddlLimeMonth"), DropDownList).Text & Chr(9)
                    Else
                        strValues = strValues & Chr(9)
                    End If

                    If CType(gvSamples.Rows(iLcv).Cells(8).FindControl("ddlLimeYear"), DropDownList).Text <> "Please select:" Then
                        strValues = strValues & CType(gvSamples.Rows(iLcv).Cells(8).FindControl("ddlLimeYear"), DropDownList).Text & Chr(9)
                    Else
                        strValues = strValues & Chr(9)
                    End If

                    strValues = strValues & vbNewLine
                Next

                'send to client end
                Response.Write(strValues)
                Response.End()