Git: Subtree Merge into a Deeply Nested Subdirectory?

I'm attempting to use git's subtree merge stategy where the subdirectory into which I want to merge is nested fairly deeply - currently four levels deep.

I followed the directions here to add the module repository as a remote, run git read-tree to get the remote code into a subdirectory in my local repo, and commit those changes.

My problem comes when I try to pull and merge changes from the remote into the master branch of my main project. Step 5 at the page above suggests a git pull with the -s subtree switch. This works correctly for me when my subdirectory is one, two or three levels deep, but not four.

Here is the result of the merge into a subdirectory 2 levels deep. You can see that the README file in sites/all/ has been updated correctly. In my remote repo, the README is in the root.

$ git pull -s subtree REMOTE_REPO master
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 2), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From /path/to/my/REMOTE_REPO
 * branch            master     -> FETCH_HEAD
Merge made by subtree.
 sites/all/README |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

Here the subdirectory is 3 levels deep: sites/all/modules/. This works fine, too, pulling the changes and updating the files.

$ git pull -s subtree REMOTE_REPO master
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 2), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From /path/to/my/REMOTE_REPO
 * branch            master     -> FETCH_HEAD
Merge made by subtree.
 sites/all/modules/README |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

But now my code is in a subdirectory 4 levels deep: sites/all/modules/my_module/. Git seems to pull the changes from REMOTE_REPO, but it doesn't update the files, instead telling me that it's already up to date.

$ git pull -s subtree REMOTE_REPO master
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 2), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From /path/to/my/REMOTE_REPO
 * branch            master     -> FETCH_HEAD
Already up-to-date!
Merge made by subtree.

And if I run it again right away, it doesn't pull the changes or update the files.

$ git pull -s subtree REMOTE_REPO master
From /path/to/my/REMOTE_REPO
 * branch            master     -> FETCH_HEAD
Already up-to-date.

Viewing the git log at this point will show me the changes from the remote repo and the merge, but the files in my checkout have not been updated.

Is this a bug, or am I doing something wrong?

Update: Chris Johnsen offered the following option, which throws an error:

$ git pull -X subtree=sites/all/modules/my_module/ REMOTE_REPO master
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 2), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From /Users/jeff/work/checkouts/compass_suite
 * branch            master     -> FETCH_HEAD
fatal: entry  not found in tree 173d4f16af4d2d61ae5c4b3446c392e8b49cc57d