wx.MessageDialog displayed without components inside

[perchef]

Is there a reason why a wx.MessageDialog would be displayed without
components inside ?

this :

md = wx.MessageDialog(None,'foo','bar',wx.YES_NO)
result = md.ShowModal()
md.Destroy()

In my application, give me :
http://img252.imageshack.us/my.php?image=messagedialog6nw.jpg

This isn't a bug in wxPython because when I use the same code in
pyshell I obtain the expected result. So, there must be a reason but I
can't figure out why.

Thanks.