Problem on gdk_window_shape_combine_mask and gtk_timeout

[Magicloud Magiclouds <magicloud magiclouds gmail com>]

Hello,
	I use gdk_window_shape_combine_mask in gtk_timeout timer, to change my 
window's shape every second.
	While I first use gdk_window_shape_combine_mask without timer, it 
works. Then put it in the timer, the application crashed, with message:

The program 'crazyclock.rb' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadMatch (invalid parameter attributes)'.
  (Details: serial 181 error_code 8 request_code 128 minor_code 2)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() 
function.)

        Could someone help me out?

Thanks.