Undo error - well it's not working!

[aikidokid]

I have the following code,but when the user clicks on the undo menu item - nothing happens.

Is there anything obvious I should or shouldn't be doing?

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, _
                    ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Const WM_USER = &H400
Private Const EM_UNDO = &HC7
 
Private Sub mnuUndo_Click()
UndoTextBox rtb1
End Sub
 
Private Sub UndoTextBox(Txtbx As RichTextBox)
    Dim X As Long
    
    X = SendMessage(Txtbx.hwnd, EM_UNDO, 0, 0)
End Sub

I am fairly new to API's so is this correct?