Bug? Subtree merge seems to choke on trailing slashes.

I'm summarizing from here: http://stackoverflow.com/questions/5904256/git-subtree-merge-into-a-deeply-nested-subdirectory

Quick repro:
1) I do an initial subtree merge in what I think is the standard way
(http://www.kernel.org/pub/software/scm/git/docs/howto/using-merge-subtree.html).
My prefix is simply "test/".
2) I try to merge more upstream changes on top of that with the
following command:
git merge --strategy-option=subtree='test/' $upstream_stuff
3) Git fails with an obscure error:
fatal: entry  not found in tree daf4d0f0a20b8b6ec007be9fcafeac84a6eba4f0

If I remove the trailing slash from the command in step 2, it works just fine: git merge --strategy-option=subtree='test' $upstream_stuff

Note in the error message above, there's a double space after "entry". Is it looking for a tree with an empty name? Did my trailing slash imply a directory named empty-string?

Thanks for your help.

-- Jack O'Connor