diff --git a/tests/test_proxy_use.py b/tests/test_proxy_use.py
index 6fdb3b47..ae6e6088 100644
--- a/tests/test_proxy_use.py
+++ b/tests/test_proxy_use.py
@@ -78,6 +78,10 @@ def setUpClass(cls):
 
     unittest_toolbox.Modified_TestCase.setUpClass()
 
+    if not six.PY2:
+      raise NotImplementedError("TestWithProxies only works with Python 2"
+                                " (proxy_server.py is Python2 only)")
+
     # Launch a simple HTTP server (serves files in the current dir).
     cls.http_port = random.randint(30000, 45000)
     cls.http_server_proc = popen_python(