Export GridView to Excel

See code below.  I have the following code to write my contents of my gridview to excel.  In one column I am sometimes getting a funny character and I'm not sure why or how to get rid of it.  I could intercept it in the rowdatabound event?  How would I find and replace this character.  Any ideas on what would cause this?

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'Write out what is in the Gridview
        Response.Clear()
        Response.Buffer = True
        Response.ContentType = "application/vnd.ms-excel"
        GridView1.Caption = "Grade Groups"
        Response.AddHeader("Content-Disposition", "attachment; filename=" + GridView1.Caption + ".xls")
        Response.Charset = ""
        Me.EnableViewState = False
        Dim oStringWriter As New System.IO.StringWriter
        Dim oHtmlTextWriter As New System.Web.UI.HtmlTextWriter(oStringWriter)
        GridView1.Caption = ""
        GridView1.FooterRow.Visible = False
        GridView1.RenderControl(oHtmlTextWriter)

        Response.Write(oStringWriter.ToString())
        Response.[End]()

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