unable to get div id from asp.net to vb.net code

Protected Sub btn_export_to_excel_Click(sender As Object, e As EventArgs)

StrExport.Append("")

StrExport.Append("

StrExport.Append(dvData.InnerHtml) //ERROR WHILE GETTING DIV ID

StrExport.Append("

Dim strFile As String = "StudentInformations_CODESCRATCHER.xls"

Dim strcontentType As String = "application/excel"

Response.ClearContent()

Response.ClearHeaders()

Response.BufferOutput = True

Response.ContentType = strcontentType

Response.AddHeader("Content-Disposition", Convert.ToString("attachment; filename=") & strFile)

Response.Write(StrExport.ToString())

Response.Flush()

Response.Close()

Response.[End]()

End Sub