Can't use file name completion in Customize for "Program Files" subdir

[Lennart Borgman]

M-x customize-option RET explicit-shell-file-name

Choose "File" in Value Menu and then type "C:/Program File/" and M-Tab. 
This gives the error

"Can't find completion for "Files/"

In GNU Emacs 22.0.50.1 (i386-mingw-nt5.0.2195)
of 2005-09-23 on W2ONE

Note: You may have to type something else instead of M-Tab to get 
"widget-complete".