编译错误：用户定义的类型未定义VB 6.0

Question

我收到此错误：

Dim frmD as frmDocument

我在Windows XP下使用VB 6.0，可以请任何人告诉我应该添加哪些参考？

这是确切的代码：

Private Sub LoadNewDoc()
    Static lDocumentCount As Long
    Dim frmD As frmDocument  // at this line i got error, User defined type not defined
    lDocumentCount = lDocumentCount + 1
    Set frmD = New frmDocument
    frmD.Caption = "Document " & lDocumentCount
    frmD.Show
End Sub