izpack2app.py fails with error "File exists"

The izpack2app.py script copies the "Mac-App-Template" to the target folder. This template contains a folder "Contents/Resources/Java/' but nevertheless the script tries to create the subdir "Java".

izpack2app.py:
...
copytree(os.path.join(base, 'Mac-App-Template'), app)
java_folder = os.path.join(app, 'Contents/Resources/Java/')
os.mkdir(java_folder)
...

Error message of my Ant script:
...
[exec] Traceback (most recent call last):
[exec] File "/Applications/Java/IzPack/utils/izpack2app/izpack2app.py", line 52, in <module>
[exec] main()
[exec] File "/Applications/Java/IzPack/utils/izpack2app/izpack2app.py", line 34, in main
[exec] os.mkdir(java_folder)
[exec] OSError: [Errno 17] File exists: '/Users/ralph/Projekte/MyApp Installer.app/Contents/Resources/Java/'
[exec] Result: 1
...