setAlpha exception

[kenshiro55]

I tried to use window transparency so I have this code:

Shell shell = new Shell(DWT.SHELL_TRIM);
shell.setVisible(true);
shell.setAlpha(100);

But I have this error: object.Exception: Access Violation.

I searched the cause and it crashed at this line in Shell.setAlpha:
OS.SetLayeredWindowAttributes (handle, 0, cast(byte)alpha, OS.LWA_ALPHA);

Is it a bug or my environment ?

Tested with dmd 1.0.28, winxp sp2, tango 0.99.6 and dwt-win 2008-07-21.