subtree merge tries to merge into wrong directory

[Nils Adermann <naderman@naderman.de>]

Following 
http://www.kernel.org/pub/software/scm/git/docs/howto/using-merge-subtree.html 
I have subtree merged a 3rd party library (ezc-reflection) into my 
repository (pflow). The prefix I used was lib/ezc/trunk/Reflection. Now 
there have been changes to ezc-reflection but merging them into my 
repository fails. The merge is attempted in example/ rather than 
lib/ezc/trunk/Reflection.

I originally set this up with:

git remote add -f ezc-reflection /path/to/ezc-reflection
git merge -s ours --no-commit ezc-reflection/master
git read-tree --prefix=lib/ezc/trunk/Reflection -u ezc-reflection/master

Reproduce my problem by executing the following:

git clone git://github.com/naderman/pflow.git
cd pflow
git reset --hard f3e001e3
git fetch git://github.com/naderman/ezc-reflection.git master
git merge -s subtree FETCH_HEAD

As you see this results in:

CONFLICT (delete/modify): example/src/doc_comment_parser.php deleted in 
HEAD and modified in FETCH_HEAD. Version FETCH_HEAD of 
example/src/doc_comment_parser.php left in tree.
Automatic merge failed; fix conflicts and then commit the result.

As long as I don't make any changes to lib/ezc/trunk/Reflection that I 
want to merge I can work around this by simply repeating my original 
process like this:

git clone git://github.com/naderman/pflow.git
cd pflow
git reset --hard f3e001e3
git fetch git://github.com/naderman/ezc-reflection.git master
git merge -s ours --no-commit FETCH_HEAD
git rm -r lib/ezc/trunk/Reflection/
git read-tree --prefix=lib/ezc/trunk/Reflection/ -u FETCH_HEAD

Why does this problem occur and how can I get the merge to work properly?

Cheers,
Nils Adermann