[QUESTION] or [BUG] Could not stash new bin files

I'm quite new to Git (using it since a few weeks only) and really like this tool. This is a great improvement over Svn that I was using until now. (Still using it for the company master repository though.)

I have the following problem:

I want to stash new files. I tried the combination "git add" / "git stash" but it does not work. Did I do something wrong, is it a special case, or some kind of bug? Does anybody have a clue?

Here are the details:

    $ git add -N XXX/src/jrxml/*.xls
    $ git stash save --keep-index "Improving performance of views"
    XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-new.xls: not added yet
    XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-old.xls: not added yet
    fatal: git-write-tree: error building trees
    Cannot save the current index state
    $ git status
    # On branch MS9
    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #       modified:   XXX/.settings/org.eclipse.wst.common.component
    #       new file:   XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-new.xls
    #       new file:   XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-old.xls
    #
    # Changed but not updated:
    #   (use "git add <file>..." to update what will be committed)
    #   (use "git checkout -- <file>..." to discard changes in working
directory)
    #
    #       modified:   XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-new.xls
    #       modified:   XXX/src/jrxml/SubCellMatrixCompared-EXPLAIN-old.xls
    #
    $ git version
    git version 1.7.0.2.msysgit.0

Specificities:

* I have a file in the index that I do not want to stash
* The two new files are binary files
* I'm running this on windows (msysgit)

Please tell me, should you need any extra information.