Python distutils error: "[directory]... doesn't exist or not a regular file"

Let's take the following project layout:

$ ls -R .
.:
package  setup.py

./package:
__init__.py  dir  file.dat  module.py

./package/dir:
tool1.dat  tool2.dat

And the following content for setup.py:

$ cat setup.py 
from distutils.core import setup


setup(name='pyproj',
      version='0.1',

      packages=[
          'package',
      ],
      package_data={
          'package': [
              '*',
              'dir/*',
          ],
      },
     )

As you can see, I want to include all non-Python files in package/ and package/dir/ directories. However, running setup.py install would raise the following error:

$ python setup.py install
running install
running build
running build_py
creating build
creating build/lib
creating build/lib/package
copying package/module.py -> build/lib/package
copying package/__init__.py -> build/lib/package
error: can't copy 'package/dir': doesn't exist or not a regular file

What gives?