diff --git a/setup.py b/setup.py
index b318fdd..f95aab5 100755
--- a/setup.py
+++ b/setup.py
@@ -56,5 +56,6 @@ See http://propka.org/ for the PROPKA web server.
             ],
         },
       zip_safe=True,
+      python_requires='>=3',
       test_suite="Tests",
 )