git log -S doesn't search stash WIPs

"git log --all -Sfoo" doesn't find strings introduced in stash WIP commits, only the stash index. That is, if I do :

$ echo foo >> file
$ git add file
$ echo bar >> file
$ git stash
Saved working directory and index state "WIP on master: 00495fc... initial"
(To restore them type "git stash apply")
HEAD is now at 00495fc... initial
$ git log --all -Sfoo
commit a8834ce80ebdf84e6579280c1fc75cdb7b4ac7c9
Author: Jonathan del Strother <me@here.com>
Date:   Fri Feb 8 10:43:44 2008 +0000

    index on master: 00495fc... initial
$ git log --all -Sbar
<this space intentionally left blank>

Is this intentional? Am I missing some git-log option that would allow me to search the stash WIPs?