...
0.0.51_square_20151002_01-py2-none-any.whl/pants/backend/python/tasks/python_task.py", line 151, in cached_chroot
    extra_requirements, executable_file_content)
  File "/Users/square/.pex/install/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl.b3f6b875b9e02db4b11b549db9cac13efaabdade/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl/pants/backend/python/tasks/python_task.py", line 186, in _build_chroot
    chroot.dump()
  File "/Users/square/.pex/install/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl.b3f6b875b9e02db4b11b549db9cac13efaabdade/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl/pants/backend/python/python_chroot.py", line 226, in dump
    distributions = self._resolve_multi(reqs_to_build, find_links)
  File "/Users/square/.pex/install/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl.b3f6b875b9e02db4b11b549db9cac13efaabdade/pantsbuild.pants-0.0.51_square_20151002_01-py2-none-any.whl/pants/backend/python/python_chroot.py", line 265, in _resolve_multi
    cache_ttl=self._python_setup.resolver_cache_ttl)
  File "/Users/square/.pex/install/pex-1.0.3-py2.py3-none-any.whl.9b479c2967fcb9866cb8b019740e265c9bc00da2/pex-1.0.3-py2.py3-none-any.whl/pex/resolver.py", line 337, in resolve
    return resolver.resolve(resolvables_from_iterable(requirements, builder))
  File "/Users/square/.pex/install/pex-1.0.3-py2.py3-none-any.whl.9b479c2967fcb9866cb8b019740e265c9bc00da2/pex-1.0.3-py2.py3-none-any.whl/pex/resolver.py", line 188, in resolve
    raise self.Error('Ambiguous resolvable: %s' % resolvable)

Exception message: Ambiguous resolvable: six>=1.4