OutOfBoundsException in ShowModal statement?

[Arnaud Nicolet <arnaud at tribu dot ch>]

Hello,

I've this code:

  w=new WinMessageToSend
  w.ShowModal
end sub

I put a breakpoint at the w.ShowModal line. Then, w is shown as  
modal. When w is closed, I get an OutOfBounds Exception "at" the  
w.ShowModal line.
What can cause a ShowModal method to generate such an exception after  
it has been executed?