请教，如何通过vb.net 获得word文档里的控件textbox 的值呢？

我在test.doc (word 2003)里添加了控件 Textbox，名称为box1，现在想给textbox里添加文字。
用下面的代码，得的结果是：
“集合所要求的成员不存在”

代码片段为：
 Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim missing = System.Reflection.Missing.Value
        Dim wApp As Word.Application
        wApp = New Word.Application
        wApp.Visible = True
        Dim wDoc As Word.Document
        wDoc = wApp.Documents.Open(FileName:="D:\Backup\我的文档\test") '(打开指定文件)
        Try
            wDoc.FormFields("box1").Result = "test"
        Catch ex As Exception
            MsgBox(ex.Message, MsgBoxStyle.Critical)  '如果出错则显示出错信息并退出
        Finally
        End Try

        wApp.Quit()
        System.Runtime.InteropServices.Marshal.ReleaseComObject(wApp)
        wApp = Nothing


    End Sub

请各位前辈指教！