show method of dialog sheet class failed - runtime error 1004

Hello,
I have a problem with a dialog box in VBA Exel.  When trying to call up
the 
dialog box, I receive the runtime error 1004: Show method of dialog sheet 
class failed. I remembet it working before, now it doesn't.  The aim is to
be 
able to record very quickly by using dropdown and edit boxes

I use following macro's:


Option Explicit
'
Global fillflag As Boolean
'
Sub dialoogstarten()

dialog1:
        Application.DialogSheets("Dialog1").Show
        If fillflag = True Then
            detail_factuur
            fillflag = False
        GoTo dialog1
        End If
End Sub
'
Sub setinvullen()
    fillflag = True
End Sub
'
The debug indicates "Application.DialogSheets("Dialog1").Show", but I
can't 
find the cause.  Can somebody help me out with this?
Would be very grateful.

-- 
joe-rio