diff --git a/setup.py b/setup.py
index <HASH>..<HASH> 100644
--- a/setup.py
+++ b/setup.py
@@ -27,7 +27,7 @@ setup(
     url='https://github.com/jwodder/javaproperties',
 
     setup_requires=['pytest-runner>=2.0,<3'],
-    install_requires=['six'],
+    install_requires=['six>=1.4.0,<2'],
     tests_require=['pytest>=2.8,<3'],
 
     extras_require={
Requires six>=<I> (in which `unichr` was added)